Multi-stage builds

Multi-stage builds are useful to anyone who has struggled to optimize Dockerfiles while keeping them easy to read and maintain.

Acknowledgment

Special thanks to Alex Ellis for granting permission to use his blog post Builder pattern vs. Multi-stage builds in Docker as the basis of the examples below.

Before multi-stage builds

One of the most challenging things about building images is keeping the image size down. Each RUN , COPY , and ADD instruction in the Dockerfile adds a layer to the image, and you need to remember to clean up any artifacts you don’t need before moving on to the next layer. To write a really efficient Dockerfile, you have traditionally needed to employ shell tricks and other logic to keep the layers as small as possible and to ensure that each layer has the artifacts it needs from the previous layer and nothing else.

It was actually very common to have one Dockerfile to use for development (which contained everything needed to build your application), and a slimmed-down one to use for production, which only contained your application and exactly what was needed to run it. This has been referred to as the “builder pattern”. Maintaining two Dockerfiles is not ideal.

Here’s an example of a build.Dockerfile and Dockerfile which adhere to the builder pattern above:

build.Dockerfile :

# syntax=docker/dockerfile:1
FROM golang:1.16
WORKDIR /go/src/github.com/alexellis/href-counter/
COPY app.go ./
RUN go get -d -v golang.org/x/net/html \
  && CGO_ENABLED=0 go build -a -installsuffix cgo -o app .

Notice that this example also artificially compresses two RUN commands together using the Bash && operator, to avoid creating an additional layer in the image. This is failure-prone and hard to maintain. It’s easy to insert another command and forget to continue the line using the \ character, for example.

Dockerfile :

# syntax=docker/dockerfile:1
FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY app ./
CMD ["./app"]

build.sh :

#!/bin/sh
echo Building alexellis2/href-counter:build
docker build -t alexellis2/href-counter:build . -f build.Dockerfile

docker container create --name extract alexellis2/href-counter:build  
docker container cp extract:/go/src/github.com/alexellis/href-counter/app ./app  
docker container rm -f extract

echo Building alexellis2/href-counter:latest
docker build --no-cache -t alexellis2/href-counter:latest .
rm ./app

When you run the build.sh script, it needs to build the first image, create a container from it to copy the artifact out, then build the second image. Both images take up room on your system and you still have the app artifact on your local disk as well.

Multi-stage builds vastly simplify this situation!

Use multi-stage builds

With multi-stage builds, you use multiple FROM statements in your Dockerfile. Each FROM instruction can use a different base, and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don’t want in the final image. To show how this works, let’s adapt the Dockerfile from the previous section to use multi-stage builds.

# syntax=docker/dockerfile:1

FROM golang:1.16
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html  
COPY app.go ./
RUN CGO_ENABLED=0 go build -a -installsuffix cgo -o app .

FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=0 /go/src/github.com/alexellis/href-counter/app ./
CMD ["./app"]

You only need the single Dockerfile. You don’t need a separate build script, either. Just run docker build .

$ docker build -t alexellis2/href-counter:latest .

The end result is the same tiny production image as before, with a significant reduction in complexity. You don’t need to create any intermediate images, and you don’t need to extract any artifacts to your local system at all.

How does it work? The second FROM instruction starts a new build stage with the alpine:latest image as its base. The COPY --from=0 line copies just the built artifact from the previous stage into this new stage. The Go SDK and any intermediate artifacts are left behind, and not saved in the final image.

Name your build stages

By default, the stages are not named, and you refer to them by their integer number, starting with 0 for the first FROM instruction. However, you can name your stages, by adding an AS <NAME> to the FROM instruction. This example improves the previous one by naming the stages and using the name in the COPY instruction. This means that even if the instructions in your Dockerfile are re-ordered later, the COPY doesn’t break.

# syntax=docker/dockerfile:1

FROM golang:1.16 AS builder
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html  
COPY app.go ./
RUN CGO_ENABLED=0 go build -a -installsuffix cgo -o app .

FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /go/src/github.com/alexellis/href-counter/app ./
CMD ["./app"]  

Stop at a specific build stage

When you build your image, you don’t necessarily need to build the entire Dockerfile including every stage. You can specify a target build stage. The following command assumes you are using the previous Dockerfile but stops at the stage named builder :

$ docker build --target builder -t alexellis2/href-counter:latest .

A few scenarios where this might be very powerful are:

• Debugging a specific build stage
• Using a debug stage with all debugging symbols or tools enabled, and a lean production stage
• Using a testing stage in which your app gets populated with test data, but building for production using a different stage which uses real data

Use an external image as a “stage”

When using multi-stage builds, you are not limited to copying from stages you created earlier in your Dockerfile. You can use the COPY --from instruction to copy from a separate image, either using the local image name, a tag available locally or on a Docker registry, or a tag ID. The Docker client pulls the image if necessary and copies the artifact from there. The syntax is:

COPY --from=nginx:latest /etc/nginx/nginx.conf /nginx.conf

Use a previous stage as a new stage

You can pick up where a previous stage left off by referring to it when using the FROM directive. For example:

# syntax=docker/dockerfile:1

FROM alpine:latest AS builder
RUN apk --no-cache add build-base

FROM builder AS build1
COPY source1.cpp source.cpp
RUN g++ -o /binary source.cpp

FROM builder AS build2
COPY source2.cpp source.cpp
RUN g++ -o /binary source.cpp

Version compatibility

Multi-stage build syntax was introduced in Docker Engine 17.05.

Differences between legacy builder and BuildKit

The legacy Docker Engine builder processes all stages of a Dockerfile leading up to the selected --target . It will build a stage even if the selected target doesn’t depend on that stage.

BuildKit only builds the stages that the target stage depends on.

For example, given the following Dockerfile:

# syntax=docker/dockerfile:1
FROM ubuntu AS base
RUN echo "base"

FROM base AS stage1
RUN echo "stage1"

FROM base AS stage2
RUN echo "stage2"

With BuildKit enabled, building the stage2 target in this Dockerfile means only base and stage2 are processed. There is no dependency on stage1 , so it’s skipped.

$ DOCKER_BUILDKIT=1 docker build --no-cache -f Dockerfile --target stage2 .
[+] Building 0.4s (7/7) FINISHED                                                                    
 => [internal] load build definition from Dockerfile                                            0.0s
 => => transferring dockerfile: 36B                                                             0.0s
 => [internal] load .dockerignore                                                               0.0s
 => => transferring context: 2B                                                                 0.0s
 => [internal] load metadata for docker.io/library/ubuntu:latest                                0.0s
 => CACHED [base 1/2] FROM docker.io/library/ubuntu                                             0.0s
 => [base 2/2] RUN echo "base"                                                                  0.1s
 => [stage2 1/1] RUN echo "stage2"                                                              0.2s
 => exporting to image                                                                          0.0s
 => => exporting layers                                                                         0.0s
 => => writing image sha256:f55003b607cef37614f607f0728e6fd4d113a4bf7ef12210da338c716f2cfd15    0.0s

On the other hand, building the same target without BuildKit results in all stages being processed:

$ DOCKER_BUILDKIT=0 docker build --no-cache -f Dockerfile --target stage2 .
Sending build context to Docker daemon  219.1kB
Step 1/6 : FROM ubuntu AS base
 ---> a7870fd478f4
Step 2/6 : RUN echo "base"
 ---> Running in e850d0e42eca
base
Removing intermediate container e850d0e42eca
 ---> d9f69f23cac8
Step 3/6 : FROM base AS stage1
 ---> d9f69f23cac8
Step 4/6 : RUN echo "stage1"
 ---> Running in 758ba6c1a9a3
stage1
Removing intermediate container 758ba6c1a9a3
 ---> 396baa55b8c3
Step 5/6 : FROM base AS stage2
 ---> d9f69f23cac8
Step 6/6 : RUN echo "stage2"
 ---> Running in bbc025b93175
stage2
Removing intermediate container bbc025b93175
 ---> 09fc3770a9c4
Successfully built 09fc3770a9c4

stage1 gets executed when BuildKit is disabled, even if stage2 does not depend on it.

Packaging your software

Dockerfile

It all starts with a Dockerfile.

Docker builds images by reading the instructions from a Dockerfile. This is a text file containing instructions that adhere to a specific format needed to assemble your application into a container image and for which you can find its specification reference in the Dockerfile reference.

Here are the most common types of instructions:

Dockerfiles are crucial inputs for image builds and can facilitate automated, multi-layer image builds based on your unique configurations. Dockerfiles can start simple and grow with your needs and support images that require complex instructions. For all the possible instructions, see the Dockerfile reference.

The default filename to use for a Dockerfile is Dockerfile , without a file extension. Using the default name allows you to run the docker build command without having to specify additional command flags.

Some projects may need distinct Dockerfiles for specific purposes. A common convention is to name these <something>.Dockerfile . Such Dockerfiles can then be used through the --file (or -f shorthand) option on the docker build command. Refer to the “Specify a Dockerfile” section in the docker build reference to learn about the --file option.

Note

We recommend using the default ( Dockerfile ) for your project’s primary Dockerfile.

Docker images consist of read-only layers , each resulting from an instruction in the Dockerfile. Layers are stacked sequentially and each one is a delta representing the changes applied to the previous layer.

Example

Here’s a simple Dockerfile example to get you started with building images. We’ll take a simple “Hello World” Python Flask application, and bundle it into a Docker image that can test locally or deploy anywhere!

Let’s say we have a hello.py file with the following content:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

Don’t worry about understanding the full example if you’re not familiar with Python, it’s just a simple web server that will contain a single page that says “Hello World”.

Note

If you test the example, make sure to copy over the indentation as well! For more information about this sample Flask application, check the Flask Quickstart page.

Here’s the Dockerfile that will be used to create an image for our application:

# syntax=docker/dockerfile:1
FROM ubuntu:22.04

# install app dependencies
RUN apt-get update && apt-get install -y python3 python3-pip
RUN pip install flask==2.1.*

# install app
COPY hello.py /

# final configuration
ENV FLASK_APP=hello
EXPOSE 8000
CMD flask run --host 0.0.0.0 --port 8000

The first line to add to a Dockerfile is a # syntax parser directive. While optional, this directive instructs the Docker builder what syntax to use when parsing the Dockerfile, and allows older Docker versions with BuildKit enabled to use a specific Dockerfile frontend before starting the build. Parser directives must appear before any other comment, whitespace, or Dockerfile instruction in your Dockerfile, and should be the first line in Dockerfiles.

# syntax=docker/dockerfile:1

Note

We recommend using docker/dockerfile:1 , which always points to the latest release of the version 1 syntax. BuildKit automatically checks for updates of the syntax before building, making sure you are using the most current version.

Next we define the first instruction:

FROM ubuntu:22.04

Here the FROM instruction sets our base image to the 22.04 release of Ubuntu. All following instructions are executed on this base image, in this case, an Ubuntu environment. The notation ubuntu:22.04 , follows the name:tag standard for naming docker images. When you build your image you use this notation to name your images and use it to specify any existing Docker image. There are many public images you can leverage in your projects. Explore Docker Hub to find out.

# install app dependencies
RUN apt-get update && apt-get install -y python3 python3-pip

This RUN instruction executes a shell command in the build context.

In this example, our context is a full Ubuntu operating system, so we have access to its package manager, apt. The provided commands update our package lists and then, after that succeeds, installs python3 and pip , the package manager for Python.

Also note # install app dependencies line. This is a comment. Comments in Dockerfiles begin with the # symbol. As your Dockerfile evolves, comments can be instrumental to document how your dockerfile works for any future readers and editors of the file.

Note

Starting your Dockerfile by a # like regular comments is treated as a directive when you are using BuildKit (default), otherwise it is ignored.

RUN pip install flask==2.1.*

This second RUN instruction requires that we’ve installed pip in the layer before. After applying the previous directive, we can use the pip command to install the flask web framework. This is the framework we’ve used to write our basic “Hello World” application from above, so to run it in Docker, we’ll need to make sure it’s installed.

COPY hello.py /

Now we use the COPY instruction to copy our hello.py file from the local build context into the root directory of our image. After being executed, we’ll end up with a file called /hello.py inside the image.

ENV FLASK_APP=hello

This ENV instruction sets a Linux environment variable we’ll need later. This is a flask-specific variable, that configures the command later used to run our hello.py application. Without this, flask wouldn’t know where to find our application to be able to run it.

EXPOSE 8000

This EXPOSE instruction marks that our final image has a service listening on port 8000 . This isn’t required, but it is a good practice, as users and tools can use this to understand what your image does.

CMD flask run --host 0.0.0.0 --port 8000

Finally, CMD instruction sets the command that is run when the user starts a container based on this image. In this case we’ll start the flask development server listening on all addresses on port 8000 .

Testing

To test our Dockerfile, we’ll first build it using the docker build command:

$ docker build -t test:latest .

Here -t test:latest option specifies the name (required) and tag (optional) of the image we’re building. . specifies the build context as the current directory. In this example, this is where build expects to find the Dockerfile and the local files the Dockerfile needs to access, in this case your Python application.

So, in accordance with the build command issued and how build context works, your Dockerfile and python app need to be in the same directory.

Now run your newly built image:

$ docker run -p 8000:8000 test:latest

From your computer, open a browser and navigate to http://localhost:8000

Note

You can also build and run using Play with Docker that provides you with a temporary Docker instance in the cloud.

Other resources

If you are interested in examples in other languages, such as Go, check out our language-specific guides in the Guides section.

Color output controls

BuildKit and Buildx have support for modifying the colors that are used to output information to the terminal. You can set the environment variable BUILDKIT_COLORS to something like run=123,20,245:error=yellow:cancel=blue:warning=white to set the colors that you would like to use:

Setting NO_COLOR to anything will disable any colorized output as recommended by no-color.org:

Note

Parsing errors will be reported but ignored. This will result in default color values being used where needed.

See also the list of pre-defined colors.

Configure BuildKit

If you create a docker-container or kubernetes builder with Buildx, you can apply a custom BuildKit configuration by passing the --config flag to the docker buildx create command.

Registry mirror

You can define a registry mirror to use for your builds. Doing so redirects BuildKit to pull images from a different hostname. The following steps exemplify defining a mirror for docker.io (Docker Hub) to mirror.gcr.io .

1. Create a TOML at /etc/buildkitd.toml with the following content:

   debug = true
   [registry."docker.io"]
     mirrors = ["mirror.gcr.io"]
   

   Note

   debug = true turns on debug requests in the BuildKit daemon, which logs a message that shows when a mirror is being used.

2. Create a docker-container builder that uses this BuildKit configuration:

   $ docker buildx create --use --bootstrap \
     --name mybuilder \
     --driver docker-container \
     --config /etc/buildkitd.toml
   

3. Build an image:

   docker buildx build --load . -f - <<EOF
   FROM alpine
   RUN echo "hello world"
   EOF
   

The BuildKit logs for this builder now shows that it uses the GCR mirror. You can tell by the fact that the response messages include the x-goog-* HTTP headers.

$ docker logs buildx_buildkit_mybuilder0

...
time="2022-02-06T17:47:48Z" level=debug msg="do request" request.header.accept="application/vnd.docker.container.image.v1+json, */*" request.header.user-agent=containerd/1.5.8+unknown request.method=GET spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=1356 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=1469 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:25:17 GMT" response.header.etag="\"774380abda8f4eae9a149e5d5d3efc83\"" response.header.expires="Sun, 06 Feb 2022 18:25:17 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:57 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788077652182 response.header.x-goog-hash="crc32c=V3DSrg==" response.header.x-goog-hash.1="md5=d0OAq9qPTq6aFJ5dXT78gw==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=1469 response.header.x-guploader-uploadid=ADPycduqQipVAXc3tzXmTzKQ2gTT6CV736B2J628smtD1iDytEyiYCgvvdD8zz9BT1J1sASUq9pW_ctUyC4B-v2jvhIxnZTlKg response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=760 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=1471 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:35:13 GMT" response.header.etag="\"35d688bd15327daafcdb4d4395e616a8\"" response.header.expires="Sun, 06 Feb 2022 18:35:13 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:12 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788032100793 response.header.x-goog-hash="crc32c=aWgRjA==" response.header.x-goog-hash.1="md5=NdaIvRUyfar8201DleYWqA==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=1471 response.header.x-guploader-uploadid=ADPycdtR-gJYwC7yHquIkJWFFG8FovDySvtmRnZBqlO3yVDanBXh_VqKYt400yhuf0XbQ3ZMB9IZV2vlcyHezn_Pu3a1SMMtiw response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg=fetch spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="do request" request.header.accept="application/vnd.docker.image.rootfs.diff.tar.gzip, */*" request.header.user-agent=containerd/1.5.8+unknown request.method=GET spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
time="2022-02-06T17:47:48Z" level=debug msg="fetch response received" response.header.accept-ranges=bytes response.header.age=1356 response.header.alt-svc="h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000,h3-Q050=\":443\"; ma=2592000,h3-Q046=\":443\"; ma=2592000,h3-Q043=\":443\"; ma=2592000,quic=\":443\"; ma=2592000; v=\"46,43\"" response.header.cache-control="public, max-age=3600" response.header.content-length=2818413 response.header.content-type=application/octet-stream response.header.date="Sun, 06 Feb 2022 17:25:17 GMT" response.header.etag="\"1d55e7be5a77c4a908ad11bc33ebea1c\"" response.header.expires="Sun, 06 Feb 2022 18:25:17 GMT" response.header.last-modified="Wed, 24 Nov 2021 21:07:06 GMT" response.header.server=UploadServer response.header.x-goog-generation=1637788026431708 response.header.x-goog-hash="crc32c=ZojF+g==" response.header.x-goog-hash.1="md5=HVXnvlp3xKkIrRG8M+vqHA==" response.header.x-goog-metageneration=1 response.header.x-goog-storage-class=STANDARD response.header.x-goog-stored-content-encoding=identity response.header.x-goog-stored-content-length=2818413 response.header.x-guploader-uploadid=ADPycdsebqxiTBJqZ0bv9zBigjFxgQydD2ESZSkKchpE0ILlN9Ibko3C5r4fJTJ4UR9ddp-UBd-2v_4eRpZ8Yo2llW_j4k8WhQ response.status="200 OK" spanID=9460e5b6e64cec91 traceID=b162d3040ddf86d6614e79c66a01a577
...

Setting registry certificates

If you specify registry certificates in the BuildKit configuration, the daemon copies the files into the container under /etc/buildkit/certs . The following steps show adding a self-signed registry certificate to the BuildKit configuration.

1. Add the following configuration to /etc/buildkitd.toml :

   # /etc/buildkitd.toml
   debug = true
   [registry."myregistry.com"]
     ca=["/etc/certs/myregistry.pem"]
     [[registry."myregistry.com".keypair]]
       key="/etc/certs/myregistry_key.pem"
       cert="/etc/certs/myregistry_cert.pem"
   

   This tells the builder to push images to the myregistry.com registry using the certificates in the specified location ( /etc/certs ).

2. Create a docker-container builder that uses this configuration:

   $ docker buildx create --use --bootstrap \
     --name mybuilder \
     --driver docker-container \
     --config /etc/buildkitd.toml
   

3. Inspect the builder’s configuration file ( /etc/buildkit/buildkitd.toml ), it shows that the certificate configuration is now configured in the builder.

   $ docker exec -it buildx_buildkit_mybuilder0 cat /etc/buildkit/buildkitd.toml
   

   debug = true
   
   [registry]
   
     [registry."myregistry.com"]
       ca = ["/etc/buildkit/certs/myregistry.com/myregistry.pem"]
   
       [[registry."myregistry.com".keypair]]
         cert = "/etc/buildkit/certs/myregistry.com/myregistry_cert.pem"
         key = "/etc/buildkit/certs/myregistry.com/myregistry_key.pem"
   

4. Verify that the certificates are inside the container:

   $ docker exec -it buildx_buildkit_mybuilder0 ls /etc/buildkit/certs/myregistry.com/
   myregistry.pem    myregistry_cert.pem   myregistry_key.pem
   

Now you can push to the registry using this builder, and it will authenticate using the certificates:

$ docker buildx build --push --tag myregistry.com/myimage:latest .

CNI networking

CNI networking for builders can be useful for dealing with network port contention during concurrent builds. CNI is not yet available in the default BuildKit image. But you can create your own image that includes CNI support.

The following Dockerfile example shows a custom BuildKit image with CNI support. It uses the CNI config for integration tests in BuildKit as an example. Feel free to include your own CNI configuration.

# syntax=docker/dockerfile:1

ARG BUILDKIT_VERSION=v{{ site.buildkit_version }}
ARG CNI_VERSION=v1.0.1

FROM --platform=$BUILDPLATFORM alpine AS cni-plugins
RUN apk add --no-cache curl
ARG CNI_VERSION
ARG TARGETOS
ARG TARGETARCH
WORKDIR /opt/cni/bin
RUN curl -Ls https://github.com/containernetworking/plugins/releases/download/$CNI_VERSION/cni-plugins-$TARGETOS-$TARGETARCH-$CNI_VERSION.tgz | tar xzv

FROM moby/buildkit:${BUILDKIT_VERSION}
ARG BUILDKIT_VERSION
RUN apk add --no-cache iptables
COPY --from=cni-plugins /opt/cni/bin /opt/cni/bin
ADD https://raw.githubusercontent.com/moby/buildkit/${BUILDKIT_VERSION}/hack/fixtures/cni.json /etc/buildkit/cni.json

Now you can build this image, and create a builder instance from it using the --driver-opt image option:

$ docker buildx build --tag buildkit-cni:local --load .
$ docker buildx create --use --bootstrap \
  --name mybuilder \
  --driver docker-container \
  --driver-opt "image=buildkit-cni:local" \
  --buildkitd-flags "--oci-worker-net=cni"

Resource limiting

Max parallelism

You can limit the parallelism of the BuildKit solver, which is particularly useful for low-powered machines, using a BuildKit configuration while creating a builder with the --config flags.

# /etc/buildkitd.toml
[worker.oci]
  max-parallelism = 4

Now you can create a docker-container builder that will use this BuildKit configuration to limit parallelism.

$ docker buildx create --use \
  --name mybuilder \
  --driver docker-container \
  --config /etc/buildkitd.toml

TCP connection limit

TCP connections are limited to 4 simultaneous connections per registry for pulling and pushing images, plus one additional connection dedicated to metadata requests. This connection limit prevents your build from getting stuck while pulling images. The dedicated metadata connection helps reduce the overall build time.

More information: moby/buildkit#2259

Custom Dockerfile syntax

Dockerfile frontend

BuildKit supports loading frontends dynamically from container images. To use an external Dockerfile frontend, the first line of your Dockerfile needs to set the syntax directive pointing to the specific image you want to use:

# syntax=[remote image reference]

For example:

# syntax=docker/dockerfile:1
# syntax=docker.io/docker/dockerfile:1
# syntax=example.com/user/repo:tag@sha256:abcdef...

This defines the location of the Dockerfile syntax that is used to build the Dockerfile. The BuildKit backend allows seamlessly using external implementations that are distributed as Docker images and execute inside a container sandbox environment.

Custom Dockerfile implementations allow you to:

• Automatically get bugfixes without updating the Docker daemon
• Make sure all users are using the same implementation to build your Dockerfile
• Use the latest features without updating the Docker daemon
• Try out new features or third-party features before they are integrated in the Docker daemon
• Use alternative build definitions, or create your own

Note

BuildKit also ships with a built-in Dockerfile frontend, but it’s recommended to use an external image to make sure that all users use the same version on the builder and to pick up bugfixes automatically without waiting for a new version of BuildKit or Docker Engine.

Official releases

Docker distributes official versions of the images that can be used for building Dockerfiles under docker/dockerfile repository on Docker Hub. There are two channels where new images are released: stable and labs .

Stable channel

The stable channel follows semantic versioning. For example:

• docker/dockerfile:1 - kept updated with the latest 1.x.x minor and patch release.
• docker/dockerfile:1.2 - kept updated with the latest 1.2.x patch release, and stops receiving updates once version 1.3.0 is released.
• docker/dockerfile:1.2.1 - immutable: never updated.

We recommend using docker/dockerfile:1 , which always points to the latest stable release of the version 1 syntax, and receives both “minor” and “patch” updates for the version 1 release cycle. BuildKit automatically checks for updates of the syntax when performing a build, making sure you are using the most current version.

If a specific version is used, such as 1.2 or 1.2.1 , the Dockerfile needs to be updated manually to continue receiving bugfixes and new features. Old versions of the Dockerfile remain compatible with the new versions of the builder.

Labs channel

The labs channel provides early access to Dockerfile features that are not yet available in the stable channel. labs images are released at the same time as stable releases, and follow the same version pattern, but use the -labs suffix, for example:

• docker/dockerfile:labs - latest release on labs channel.
• docker/dockerfile:1-labs - same as dockerfile:1 , with experimental features enabled.
• docker/dockerfile:1.2-labs - same as dockerfile:1.2 , with experimental features enabled.
• docker/dockerfile:1.2.1-labs - immutable: never updated. Same as dockerfile:1.2.1 , with experimental features enabled.

Choose a channel that best fits your needs. If you want to benefit from new features, use the labs channel. Images in the labs channel contain all the features in the stable channel, plus early access features. Stable features in the labs channel follow semantic versioning, but early access features don’t, and newer releases may not be backwards compatible. Pin the version to avoid having to deal with breaking changes.

Other resources

For documentation on “labs” features, master builds, and nightly feature releases, refer to the description in the BuildKit source repository on GitHub. For a full list of available images, visit the docker/dockerfile repository on Docker Hub, and the docker/dockerfile-upstream repository on Docker Hub for development builds.