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moby / buildkit

concurrent, cache-efficient, and Dockerfile-agnostic builder toolkit
https://github.com/moby/moby/issues/34227
Apache License 2.0
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builder buildkit cloud-native containers docker dockerfile go golang oci oci-image

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BuildKit

GitHub Release PkgGoDev CI BuildKit Status CI Frontend Status Go Report Card Codecov

BuildKit is a toolkit for converting source code to build artifacts in an efficient, expressive and repeatable manner.

Key features:

Read the proposal from https://github.com/moby/moby/issues/32925

Introductory blog post https://blog.mobyproject.org/introducing-buildkit-17e056cc5317

Join #buildkit channel on Docker Community Slack

Note

If you are visiting this repo for the usage of BuildKit-only Dockerfile features like RUN --mount=type=(bind|cache|tmpfs|secret|ssh), please refer to the Dockerfile reference.

Note

docker build uses Buildx and BuildKit by default since Docker Engine 23.0. You don't need to read this document unless you want to use the full-featured standalone version of BuildKit.

Used by

BuildKit is used by the following projects:

Quick start

:information_source: For Kubernetes deployments, see examples/kubernetes.

BuildKit is composed of the buildkitd daemon and the buildctl client. While the buildctl client is available for Linux, macOS, and Windows, the buildkitd daemon is only available for Linux and *Windows currently.

The latest binaries of BuildKit are available here for Linux, macOS, and Windows.

Linux Setup

The buildkitd daemon requires the following components to be installed:

Starting the buildkitd daemon: You need to run buildkitd as the root user on the host.

$ sudo buildkitd

To run buildkitd as a non-root user, see docs/rootless.md.

The buildkitd daemon supports two worker backends: OCI (runc) and containerd.

By default, the OCI (runc) worker is used. You can set --oci-worker=false --containerd-worker=true to use the containerd worker.

We are open to adding more backends.

To start the buildkitd daemon using systemd socket activation, you can install the buildkit systemd unit files. See Systemd socket activation

The buildkitd daemon listens gRPC API on /run/buildkit/buildkitd.sock by default, but you can also use TCP sockets. See Expose BuildKit as a TCP service.

Windows Setup

See instructions and notes at docs/windows.md.

macOS Setup

Homebrew formula (unofficial) is available for macOS.

$ brew install buildkit

The Homebrew formula does not contain the daemon (buildkitd).

For example, Lima can be used for launching the daemon inside a Linux VM.

brew install lima
limactl start template://buildkit
export BUILDKIT_HOST="unix://$HOME/.lima/buildkit/sock/buildkitd.sock"

Build from source

To build BuildKit from source, see .github/CONTRIBUTING.md.

For a buildctl reference, see this document.

Exploring LLB

BuildKit builds are based on a binary intermediate format called LLB that is used for defining the dependency graph for processes running part of your build. tl;dr: LLB is to Dockerfile what LLVM IR is to C.

See solver/pb/ops.proto for the format definition, and see ./examples/README.md for example LLB applications.

Currently, the following high-level languages have been implemented for LLB:

Exploring Dockerfiles

Frontends are components that run inside BuildKit and convert any build definition to LLB. There is a special frontend called gateway (gateway.v0) that allows using any image as a frontend.

During development, Dockerfile frontend (dockerfile.v0) is also part of the BuildKit repo. In the future, this will be moved out, and Dockerfiles can be built using an external image.

Building a Dockerfile with buildctl

buildctl build \
    --frontend=dockerfile.v0 \
    --local context=. \
    --local dockerfile=.
# or
buildctl build \
    --frontend=dockerfile.v0 \
    --local context=. \
    --local dockerfile=. \
    --opt target=foo \
    --opt build-arg:foo=bar

--local exposes local source files from client to the builder. context and dockerfile are the names Dockerfile frontend looks for build context and Dockerfile location.

If the Dockerfile has a different filename it can be specified with --opt filename=./Dockerfile-alternative.

Building a Dockerfile using external frontend

External versions of the Dockerfile frontend are pushed to https://hub.docker.com/r/docker/dockerfile-upstream and https://hub.docker.com/r/docker/dockerfile and can be used with the gateway frontend. The source for the external frontend is currently located in ./frontend/dockerfile/cmd/dockerfile-frontend but will move out of this repository in the future (#163). For automatic build from master branch of this repository docker/dockerfile-upstream:master or docker/dockerfile-upstream:master-labs image can be used.

buildctl build \
    --frontend gateway.v0 \
    --opt source=docker/dockerfile \
    --local context=. \
    --local dockerfile=.
buildctl build \
    --frontend gateway.v0 \
    --opt source=docker/dockerfile \
    --opt context=https://github.com/moby/moby.git \
    --opt build-arg:APT_MIRROR=cdn-fastly.deb.debian.org

Output

By default, the build result and intermediate cache will only remain internally in BuildKit. An output needs to be specified to retrieve the result.

Image/Registry

buildctl build ... --output type=image,name=docker.io/username/image,push=true

To export the image to multiple registries:

buildctl build ... --output type=image,\"name=docker.io/username/image,docker.io/username2/image2\",push=true

To export the cache embed with the image and pushing them to registry together, type registry is required to import the cache, you should specify --export-cache type=inline and --import-cache type=registry,ref=.... To export the cache to a local directly, you should specify --export-cache type=local. Details in Export cache.

buildctl build ...\
  --output type=image,name=docker.io/username/image,push=true \
  --export-cache type=inline \
  --import-cache type=registry,ref=docker.io/username/image

Keys supported by image output:

If credentials are required, buildctl will attempt to read Docker configuration file $DOCKER_CONFIG/config.json. $DOCKER_CONFIG defaults to ~/.docker.

Local directory

The local client will copy the files directly to the client. This is useful if BuildKit is being used for building something else than container images.

buildctl build ... --output type=local,dest=path/to/output-dir

To export specific files use multi-stage builds with a scratch stage and copy the needed files into that stage with COPY --from.

...
FROM scratch as testresult

COPY --from=builder /usr/src/app/testresult.xml .
...
buildctl build ... --opt target=testresult --output type=local,dest=path/to/output-dir

With a multi-platform build, a subfolder matching each target platform will be created in the destination directory:

FROM busybox AS build
ARG TARGETOS
ARG TARGETARCH
RUN mkdir /out && echo foo > /out/hello-$TARGETOS-$TARGETARCH

FROM scratch
COPY --from=build /out /
$ buildctl build \
  --frontend dockerfile.v0 \
  --opt platform=linux/amd64,linux/arm64 \
  --output type=local,dest=./bin/release

$ tree ./bin
./bin/
└── release
    ├── linux_amd64
    │   └── hello-linux-amd64
    └── linux_arm64
        └── hello-linux-arm64

You can set platform-split=false to merge files from all platforms together into same directory:

$ buildctl build \
  --frontend dockerfile.v0 \
  --opt platform=linux/amd64,linux/arm64 \
  --output type=local,dest=./bin/release,platform-split=false

$ tree ./bin
./bin/
└── release
    ├── hello-linux-amd64
    └── hello-linux-arm64

Tar exporter is similar to local exporter but transfers the files through a tarball.

buildctl build ... --output type=tar,dest=out.tar
buildctl build ... --output type=tar > out.tar

Docker tarball

# exported tarball is also compatible with OCI spec
buildctl build ... --output type=docker,name=myimage | docker load

OCI tarball

buildctl build ... --output type=oci,dest=path/to/output.tar
buildctl build ... --output type=oci > output.tar

containerd image store

The containerd worker needs to be used

buildctl build ... --output type=image,name=docker.io/username/image
ctr --namespace=buildkit images ls

To change the containerd namespace, you need to change worker.containerd.namespace in /etc/buildkit/buildkitd.toml.

Cache

To show local build cache (/var/lib/buildkit):

buildctl du -v

To prune local build cache:

buildctl prune

Garbage collection

See ./docs/buildkitd.toml.md.

Export cache

BuildKit supports the following cache exporters:

In most case you want to use the inline cache exporter. However, note that the inline cache exporter only supports min cache mode. To enable max cache mode, push the image and the cache separately by using registry cache exporter.

inline and registry exporters both store the cache in the registry. For importing the cache, type=registry is sufficient for both, as specifying the cache format is not necessary.

Inline (push image and cache together)

buildctl build ... \
  --output type=image,name=docker.io/username/image,push=true \
  --export-cache type=inline \
  --import-cache type=registry,ref=docker.io/username/image

Note that the inline cache is not imported unless --import-cache type=registry,ref=... is provided.

Inline cache embeds cache metadata into the image config. The layers in the image will be left untouched compared to the image with no cache information.

:information_source: Docker-integrated BuildKit (DOCKER_BUILDKIT=1 docker build) and docker buildxrequires --build-arg BUILDKIT_INLINE_CACHE=1 to be specified to enable the inline cache exporter. However, the standalone buildctl does NOT require --opt build-arg:BUILDKIT_INLINE_CACHE=1 and the build-arg is simply ignored.

Registry (push image and cache separately)

buildctl build ... \
  --output type=image,name=localhost:5000/myrepo:image,push=true \
  --export-cache type=registry,ref=localhost:5000/myrepo:buildcache \
  --import-cache type=registry,ref=localhost:5000/myrepo:buildcache

--export-cache options:

--import-cache options:

Local directory

buildctl build ... --export-cache type=local,dest=path/to/output-dir
buildctl build ... --import-cache type=local,src=path/to/input-dir

The directory layout conforms to OCI Image Spec v1.0.

--export-cache options:

--import-cache options:

GitHub Actions cache (experimental)

buildctl build ... \
  --output type=image,name=docker.io/username/image,push=true \
  --export-cache type=gha \
  --import-cache type=gha

GitHub Actions cache saves both cache metadata and layers to GitHub's Cache service. This cache currently has a size limit of 10GB that is shared across different caches in the repo. If you exceed this limit, GitHub will save your cache but will begin evicting caches until the total size is less than 10 GB. Recycling caches too often can result in slower runtimes overall.

Similarly to using actions/cache, caches are scoped by branch, with the default and target branches being available to every branch.

Following attributes are required to authenticate against the GitHub Actions Cache service API:

:information_source: This type of cache can be used with Docker Build Push Action where url and token will be automatically set. To use this backend in an inline run step, you have to include crazy-max/ghaction-github-runtime in your workflow to expose the runtime.

--export-cache options:

--import-cache options:

S3 cache (experimental)

buildctl build ... \
  --output type=image,name=docker.io/username/image,push=true \
  --export-cache type=s3,region=eu-west-1,bucket=my_bucket,name=my_image \
  --import-cache type=s3,region=eu-west-1,bucket=my_bucket,name=my_image

The following attributes are required:

Storage locations:

S3 configuration:

AWS Authentication:

The simplest way is to use an IAM Instance profile. Other options are:

--export-cache options:

--import-cache options:

Azure Blob Storage cache (experimental)

buildctl build ... \
  --output type=image,name=docker.io/username/image,push=true \
  --export-cache type=azblob,account_url=https://myaccount.blob.core.windows.net,name=my_image \
  --import-cache type=azblob,account_url=https://myaccount.blob.core.windows.net,name=my_image

The following attributes are required:

Storage locations:

Azure Blob Storage configuration:

Azure Blob Storage authentication:

There are 2 options supported for Azure Blob Storage authentication:

Note

Account name can also be specified with account_name attribute (or $BUILDKIT_AZURE_STORAGE_ACCOUNT_NAME) if it is not part of the account URL host.

--export-cache options:

--import-cache options:

Consistent hashing

If you have multiple BuildKit daemon instances, but you don't want to use registry for sharing cache across the cluster, consider client-side load balancing using consistent hashing.

See ./examples/kubernetes/consistenthash.

Metadata

To output build metadata such as the image digest, pass the --metadata-file flag. The metadata will be written as a JSON object to the specified file. The directory of the specified file must already exist and be writable.

buildctl build ... --metadata-file metadata.json
jq '.' metadata.json
{
  "containerimage.config.digest": "sha256:2937f66a9722f7f4a2df583de2f8cb97fc9196059a410e7f00072fc918930e66",
  "containerimage.descriptor": {
    "annotations": {
      "config.digest": "sha256:2937f66a9722f7f4a2df583de2f8cb97fc9196059a410e7f00072fc918930e66",
      "org.opencontainers.image.created": "2022-02-08T21:28:03Z"
    },
    "digest": "sha256:19ffeab6f8bc9293ac2c3fdf94ebe28396254c993aea0b5a542cfb02e0883fa3",
    "mediaType": "application/vnd.oci.image.manifest.v1+json",
    "size": 506
  },
  "containerimage.digest": "sha256:19ffeab6f8bc9293ac2c3fdf94ebe28396254c993aea0b5a542cfb02e0883fa3"
}

Systemd socket activation

On Systemd based systems, you can communicate with the daemon via Systemd socket activation, use buildkitd --addr fd://. You can find examples of using Systemd socket activation with BuildKit and Systemd in ./examples/systemd.

Expose BuildKit as a TCP service

The buildkitd daemon can listen the gRPC API on a TCP socket.

It is highly recommended to create TLS certificates for both the daemon and the client (mTLS). Enabling TCP without mTLS is dangerous because the executor containers (aka Dockerfile RUN containers) can call BuildKit API as well.

buildkitd \
  --addr tcp://0.0.0.0:1234 \
  --tlscacert /path/to/ca.pem \
  --tlscert /path/to/cert.pem \
  --tlskey /path/to/key.pem
buildctl \
  --addr tcp://example.com:1234 \
  --tlscacert /path/to/ca.pem \
  --tlscert /path/to/clientcert.pem \
  --tlskey /path/to/clientkey.pem \
  build ...

Load balancing

buildctl build can be called against randomly load balanced buildkitd daemons.

See also Consistent hashing for client-side load balancing.

Containerizing BuildKit

BuildKit can also be used by running the buildkitd daemon inside a Docker container and accessing it remotely.

We provide the container images as moby/buildkit:

To run daemon in a container:

docker run -d --name buildkitd --privileged moby/buildkit:latest
export BUILDKIT_HOST=docker-container://buildkitd
buildctl build --help

Podman

To connect to a BuildKit daemon running in a Podman container, use podman-container:// instead of docker-container:// .

podman run -d --name buildkitd --privileged moby/buildkit:latest
buildctl --addr=podman-container://buildkitd build --frontend dockerfile.v0 --local context=. --local dockerfile=. --output type=oci | podman load foo

sudo is not required.

Nerdctl

To connect to a BuildKit daemon running in a Nerdctl container, use nerdctl-container:// instead of docker-container://.

nerdctl run -d --name buildkitd --privileged moby/buildkit:latest
buildctl --addr=nerdctl-container://buildkitd build --frontend dockerfile.v0 --local context=. --local dockerfile=. --output type=oci | nerdctl load

sudo is not required.

Kubernetes

For Kubernetes deployments, see examples/kubernetes.

Daemonless

To run the client and an ephemeral daemon in a single container ("daemonless mode"):

docker run \
    -it \
    --rm \
    --privileged \
    -v /path/to/dir:/tmp/work \
    --entrypoint buildctl-daemonless.sh \
    moby/buildkit:master \
        build \
        --frontend dockerfile.v0 \
        --local context=/tmp/work \
        --local dockerfile=/tmp/work

or

docker run \
    -it \
    --rm \
    --security-opt seccomp=unconfined \
    --security-opt apparmor=unconfined \
    -e BUILDKITD_FLAGS=--oci-worker-no-process-sandbox \
    -v /path/to/dir:/tmp/work \
    --entrypoint buildctl-daemonless.sh \
    moby/buildkit:master-rootless \
        build \
        --frontend \
        dockerfile.v0 \
        --local context=/tmp/work \
        --local dockerfile=/tmp/work

OpenTelemetry support

BuildKit supports OpenTelemetry for buildkitd gRPC API and buildctl commands. To capture the trace to Jaeger, set JAEGER_TRACE environment variable to the collection address.

docker run -d -p6831:6831/udp -p16686:16686 jaegertracing/all-in-one:latest
export JAEGER_TRACE=0.0.0.0:6831
# restart buildkitd and buildctl so they know JAEGER_TRACE
# any buildctl command should be traced to http://127.0.0.1:16686/

On Windows, if you are running Jaeger outside of a container, jaeger-all-in-one.exe, set the environment variable setx -m JAEGER_TRACE "0.0.0.0:6831", restart buildkitd in a new terminal and the traces will be collected automatically.

Running BuildKit without root privileges

Please refer to docs/rootless.md.

Building multi-platform images

Please refer to docs/multi-platform.md.

Configuring buildctl

Color Output Controls

buildctl has 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=green:warning=yellow:error=red:cancel=255,165,0 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.

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

Number of log lines (for active steps in tty mode)

You can change how many log lines are visible for active steps in tty mode by setting BUILDKIT_TTY_LOG_LINES to a number (default: 6).

Contributing

Want to contribute to BuildKit? Awesome! You can find information about contributing to this project in the CONTRIBUTING.md