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

Docker driver

The Buildx Docker driver is the default driver. It uses the BuildKit server components built directly into the Docker engine. The Docker driver requires no configuration.

Unlike the other drivers, builders using the Docker driver can’t be manually created. They’re only created automatically from the Docker context.

Images built with the Docker driver are automatically loaded to the local image store.

Synopsis

# The Docker driver is used by buildx by default
docker buildx build .

It’s not possible to configure which BuildKit version to use, or to pass any additional BuildKit parameters to a builder using the Docker driver. The BuildKit version and parameters are preset by the Docker engine internally.

If you need additional configuration and flexibility, consider using the Docker container driver.

Further reading

For more information on the Docker driver, see the buildx reference.

Create a base image

Most Dockerfiles start from a parent image. If you need to completely control the contents of your image, you might need to create a base image instead. Here’s the difference:

• A parent image is the image that your image is based on. It refers to the contents of the FROM directive in the Dockerfile. Each subsequent declaration in the Dockerfile modifies this parent image. Most Dockerfiles start from a parent image, rather than a base image. However, the terms are sometimes used interchangeably.

• A base image has FROM scratch in its Dockerfile.

This topic shows you several ways to create a base image. The specific process will depend heavily on the Linux distribution you want to package. We have some examples below, and you are encouraged to submit pull requests to contribute new ones.

Create a full image using tar

In general, start with a working machine that is running the distribution you’d like to package as a parent image, though that is not required for some tools like Debian’s Debootstrap, which you can also use to build Ubuntu images.

It can be as simple as this to create an Ubuntu parent image:

$ sudo debootstrap focal focal > /dev/null
$ sudo tar -C focal -c . | docker import - focal

sha256:81ec9a55a92a5618161f68ae691d092bf14d700129093158297b3d01593f4ee3

$ docker run focal cat /etc/lsb-release

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04 LTS"

There are more example scripts for creating parent images in the Docker GitHub repository.

Create a simple parent image using scratch

You can use Docker’s reserved, minimal image, scratch , as a starting point for building containers. Using the scratch “image” signals to the build process that you want the next command in the Dockerfile to be the first filesystem layer in your image.

While scratch appears in Docker’s repository on the hub, you can’t pull it, run it, or tag any image with the name scratch . Instead, you can refer to it in your Dockerfile . For example, to create a minimal container using scratch :

# syntax=docker/dockerfile:1
FROM scratch
ADD hello /
CMD ["/hello"]

Assuming you built the “hello” executable example by using the source code at https://github.com/docker-library/hello-world, and you compiled it with the -static flag, you can build this Docker image using this docker build command:

$ docker build --tag hello .

Don’t forget the . character at the end, which sets the build context to the current directory.

Note : Because Docker Desktop for Mac and Docker Desktop for Windows use a Linux VM, you need a Linux binary, rather than a Mac or Windows binary. You can use a Docker container to build it:

$ docker run --rm -it -v $PWD:/build ubuntu:20.04

container# apt-get update && apt-get install build-essential
container# cd /build
container# gcc -o hello -static hello.c

To run your new image, use the docker run command:

$ docker run --rm hello

This example creates the hello-world image used in the tutorials. If you want to test it out, you can clone the image repo.

More resources

There are lots of resources available to help you write your Dockerfile .

• There’s a complete guide to all the instructions available for use in a Dockerfile in the reference section.
• To help you write a clear, readable, maintainable Dockerfile , we’ve also written a Dockerfile best practices guide.
• If your goal is to create a new Docker Official Image, read Docker Official Images.

Build context

The docker build or docker buildx build commands build Docker images from a Dockerfile and a “context”.

A build’s context is the set of files located at the PATH or URL specified as the positional argument to the build command:

$ docker build [OPTIONS] PATH | URL | -
                         ^^^^^^^^^^^^^^

The build process can refer to any of the files in the context. For example, your build can use a COPY instruction to reference a file in the context or a RUN --mount=type=bind instruction for better performance with BuildKit. The build context is processed recursively. So, a PATH includes any subdirectories and the URL includes the repository and its submodules.

PATH context

This example shows a build command that uses the current directory ( . ) as a build context:

$ docker build .
...
#16 [internal] load build context
#16 sha256:23ca2f94460dcbaf5b3c3edbaaa933281a4e0ea3d92fe295193e4df44dc68f85
#16 transferring context: 13.16MB 2.2s done
...

With the following Dockerfile:

# syntax=docker/dockerfile:1
FROM busybox
WORKDIR /src
COPY foo .

And this directory structure:

.
├── Dockerfile
├── bar
├── foo
└── node_modules

The legacy builder sends the entire directory to the daemon, including bar and node_modules directories, even though the Dockerfile does not use them. When using BuildKit, the client only sends the files required by the COPY instructions, in this case foo .

In some cases you may want to send the entire context:

# syntax=docker/dockerfile:1
FROM busybox
WORKDIR /src
COPY . .

You can use a .dockerignore file to exclude some files or directories from being sent:

# .dockerignore
node_modules
bar

Warning

Avoid using your root directory, / , as the PATH for your build context, as it causes the build to transfer the entire contents of your hard drive to the daemon.

URL context

The URL parameter can refer to three kinds of resources:

• Git repositories
• Pre-packaged tarball contexts
• Plain text files

Git repositories

When the URL parameter points to the location of a Git repository, the repository acts as the build context. The builder recursively pulls the repository and its submodules. A shallow clone is performed and therefore pulls down just the latest commits, not the entire history. A repository is first pulled into a temporary directory on your host. After that succeeds, the directory is sent to the daemon as the context. Local copy gives you the ability to access private repositories using local user credentials, VPN’s, and so forth.

Note

If the URL parameter contains a fragment the system will recursively clone the repository and its submodules using a git clone --recursive command.

Git URLs accept a context configuration parameter in the form of a URL fragment, separated by a colon ( : ). The first part represents the reference that Git will check out, and can be either a branch, a tag, or a remote reference. The second part represents a subdirectory inside the repository that will be used as a build context.

For example, run this command to use a directory called docker in the branch container :

$ docker build https://github.com/user/myrepo.git#container:docker

The following table represents all the valid suffixes with their build contexts:

By default .git directory is not kept on Git checkouts. You can set the BuildKit built-in arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 to keep it. It can be useful to keep it around if you want to retrieve Git information during your build:

# syntax=docker/dockerfile:1
FROM alpine
WORKDIR /src
RUN --mount=target=. \
  make REVISION=$(git rev-parse HEAD) build

$ docker build --build-arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 https://github.com/user/myrepo.git#main

Tarball contexts

If you pass a URL to a remote tarball, the URL itself is sent to the daemon:

$ docker build http://server/context.tar.gz
#1 [internal] load remote build context
#1 DONE 0.2s

#2 copy /context /
#2 DONE 0.1s
...

The download operation will be performed on the host the daemon is running on, which is not necessarily the same host from which the build command is being issued. The daemon will fetch context.tar.gz and use it as the build context. Tarball contexts must be tar archives conforming to the standard tar UNIX format and can be compressed with any one of the xz , bzip2 , gzip or identity (no compression) formats.

Text files

Instead of specifying a context, you can pass a single Dockerfile in the URL or pipe the file in via STDIN . To pipe a Dockerfile from STDIN :

$ docker build - < Dockerfile

With Powershell on Windows, you can run:

Get-Content Dockerfile | docker build -

If you use STDIN or specify a URL pointing to a plain text file, the system places the contents into a file called Dockerfile , and any -f , --file option is ignored. In this scenario, there is no context.

The following example builds an image using a Dockerfile that is passed through stdin. No files are sent as build context to the daemon.

docker build -t myimage:latest -<<EOF
FROM busybox
RUN echo "hello world"
EOF

Omitting the build context can be useful in situations where your Dockerfile does not require files to be copied into the image, and improves the build-speed, as no files are sent to the daemon.