Dockerfile frontend¶

About¶

External versions of the Dockerfile frontend are pushed to docker/dockerfile-upstream and docker/dockerfile-upstream and can be used with the gateway frontend.

The source for the external frontend is currently located in ./frontend/dockerfile but will move out of this repository in the future (moby/buildkit#163).

For automatic build from master branch of this repository docker/dockerfile-upstream:master or docker/dockerfile-upstream:master-labs image can be used.

Note for Docker users

If you are using Docker v18.09 or later, BuildKit mode can be enabled by setting export DOCKER_BUILDKIT=1 on the client side.

Docker Buildx always enables BuildKit.

Build a Dockerfile with an external Dockerfile frontend¶

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

Using an external Dockerfile frontend¶

BuildKit supports loading frontends dynamically from container images. Images for Dockerfile frontends are available at docker/dockerfile repository.

To use the external frontend, the first line of your Dockerfile needs to be # syntax=docker/dockerfile:1.3 pointing to the specific image you want to use.

BuildKit also ships with Dockerfile frontend builtin, but it is 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.

The images are published on two channels: latest and labs. The latest channel uses semver versioning while labs uses an incrementing number. This means the labs channel may remove a feature without incrementing the major component of a version, and you may want to pin the image to a specific revision. Even when syntaxes change in between releases on labs channel, the old versions are guaranteed to be backward compatible.

Syntax¶

This section documents new BuildKit-only commands added to the Dockerfile frontend.

Build mounts `RUN --mount=...`¶

To use this flag set Dockerfile version to at least 1.2

# syntax=docker/dockerfile:1.3

RUN --mount allows you to create mounts that process running as part of the build can access. This can be used to bind files from other part of the build without copying, accessing build secrets or ssh-agent sockets, or creating cache locations to speed up your build.

`RUN --mount=type=bind`¶

Info

type=bind is the default mount type being used if not provided

This mount type allows binding directories (read-only) in the context or in an image to the build container.

Option	Description
`target`¹	Mount path.
`source`	Source path in the `from`. Defaults to the root of the `from`.
`from`	Build stage or image name for the root of the source. Defaults to the build context.
`rw`,`readwrite`	Allow writes on the mount. Written data will be discarded.

`RUN --mount=type=cache`¶

This mount type allows the build container to cache directories for compilers and package managers.

Option	Description
`id`	Optional ID to identify separate/different caches. Defaults to value of `target`.
`target`¹	Mount path.
`ro`,`readonly`	Read-only if set.
`sharing`	One of `shared`, `private`, or `locked`. Defaults to `shared`. A `shared` cache mount can be used concurrently by multiple writers. `private` creates a new mount if there are multiple writers. `locked` pauses the second writer until the first one releases the mount.
`from`	Build stage to use as a base of the cache mount. Defaults to empty directory.
`source`	Subpath in the `from` to mount. Defaults to the root of the `from`.
`mode`	File mode for new cache directory in octal. Default `0755`.
`uid`	User ID for new cache directory. Default `0`.
`gid`	Group ID for new cache directory. Default `0`.

Contents of the cache directories persists between builder invocations without invalidating the instruction cache. Cache mounts should only be used for better performance. Your build should work with any contents of the cache directory as another build may overwrite the files or GC may clean it if more storage space is needed.

Example: cache Go packages¶

# syntax=docker/dockerfile:1.3
FROM golang
RUN --mount=type=cache,target=/root/.cache/go-build \
  go build ...

Example: cache apt packages¶

# syntax=docker/dockerfile:1.3
FROM ubuntu
RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
RUN --mount=type=cache,target=/var/cache/apt \
  --mount=type=cache,target=/var/lib/apt \
  apt update && apt-get --no-install-recommends install -y gcc

`RUN --mount=type=tmpfs`¶

This mount type allows mounting tmpfs in the build container.

Option	Description
`target`¹	Mount path.
`size`	Specify an upper limit on the size of the filesystem.

`RUN --mount=type=secret`¶

This mount type allows the build container to access secure files such as private keys without baking them into the image.

Option	Description
`id`	ID of the secret. Defaults to basename of the target path.
`target`	Mount path. Defaults to `/run/secrets/` + `id`.
`required`	If set to `true`, the instruction errors out when the secret is unavailable. Defaults to `false`.
`mode`	File mode for secret file in octal. Default `0400`.
`uid`	User ID for secret file. Default `0`.
`gid`	Group ID for secret file. Default `0`.

Example: access to S3¶

# syntax=docker/dockerfile:1.3
FROM python:3
RUN pip install awscli
RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
  aws s3 cp s3://... ...

docker build --secret id=aws,src=$HOME/.aws/credentials .

buildctl build --frontend=dockerfile.v0 --local context=. --local dockerfile=. \
  --secret id=aws,src=$HOME/.aws/credentials

`RUN --mount=type=ssh`¶

This mount type allows the build container to access SSH keys via SSH agents, with support for passphrases.

Option	Description
`id`	ID of SSH agent socket or key. Defaults to "default".
`target`	SSH agent socket path. Defaults to `/run/buildkit/ssh_agent.${N}`.
`required`	If set to `true`, the instruction errors out when the key is unavailable. Defaults to `false`.
`mode`	File mode for socket in octal. Default `0600`.
`uid`	User ID for socket. Default `0`.
`gid`	Group ID for socket. Default `0`.

Example: access to Gitlab¶

# syntax=docker/dockerfile:1.3
FROM alpine
RUN apk add --no-cache openssh-client
RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
RUN --mount=type=ssh ssh -q -T git@gitlab.com 2>&1 | tee /hello
# "Welcome to GitLab, @GITLAB_USERNAME_ASSOCIATED_WITH_SSHKEY" should be printed here
# with the type of build progress is defined as `plain`.

$ eval $(ssh-agent)
$ ssh-add ~/.ssh/id_rsa
(Input your passphrase here)
$ docker build --ssh default=$SSH_AUTH_SOCK .

buildctl build --frontend=dockerfile.v0 --local context=. --local dockerfile=. \
  --ssh default=$SSH_AUTH_SOCK

You can also specify a path to *.pem file on the host directly instead of $SSH_AUTH_SOCK. However, pem files with passphrases are not supported.

Network modes `RUN --network=...`¶

# syntax=docker/dockerfile:1.3

RUN --network allows control over which networking environment the command is run in.

`RUN --network=none`¶

The command is run with no network access (lo is still available, but is isolated to this process)

Example: isolating external effects¶

# syntax=docker/dockerfile:1.3
FROM python:3.6
ADD mypackage.tgz wheels/
RUN --network=none pip install --find-links wheels mypackage

pip will only be able to install the packages provided in the tarfile, which can be controlled by an earlier build stage.

`RUN --network=host`¶

The command is run in the host's network environment (similar to docker build --network=host, but on a per-instruction basis)

Caution

The use of --network=host is protected by the network.host entitlement, which needs to be enabled when starting the buildkitd daemon (--allow-insecure-entitlement network.host) and on the build request (--allow network.host).

`RUN --network=default`¶

Equivalent to not supplying a flag at all, the command is run in the default network for the build.

Security context `RUN --security=...`¶

To use this flag, set Dockerfile version to labs channel.

# syntax=docker/dockerfile:1.3-labs

`RUN --security=insecure`¶

With --security=insecure, builder runs the command without sandbox in insecure mode, which allows to run flows requiring elevated privileges (e.g. containerd). This is equivalent to running docker run --privileged.

Caution

In order to access this feature, entitlement security.insecure should be enabled when starting the buildkitd daemon (--allow-insecure-entitlement security.insecure) and for a build request (--allow security.insecure).

Example: check entitlements¶

# syntax=docker/dockerfile:1.3-labs
FROM ubuntu
RUN --security=insecure cat /proc/self/status | grep CapEff

#84 0.093 CapEff:   0000003fffffffff

`RUN --security=sandbox`¶

Default sandbox mode can be activated via --security=sandbox, but that is no-op.

Here-Documents¶

This feature is available since docker/dockerfile:1.4.0 release.

# syntax=docker/dockerfile:1.4

Here-documents allow redirection of subsequent Dockerfile lines to the input of RUN or COPY commands. If such command contains a here-document Dockerfile will consider the next lines until the line only containing a here-doc delimiter as part of the same command.

Example: running a multi-line script¶

# syntax = docker/dockerfile:1.4
FROM debian
RUN <<eot bash
  apt-get update
  apt-get install -y vim
eot

If the command only contains a here-document, its contents is evaluated with the default shell.

# syntax = docker/dockerfile:1.4
FROM debian
RUN <<eot
  mkdir -p foo/bar
eot

Alternatively, shebang header can be used to define an interpreter.

# syntax = docker/dockerfile:1.4
FROM python:3.6
RUN <<eot
#!/usr/bin/env python
print("hello world")
eot

More complex examples may use multiple here-documents.

# syntax = docker/dockerfile:1.4
FROM alpine
RUN <<FILE1 cat > file1 && <<FILE2 cat > file2
I am
first
FILE1
I am
second
FILE2

Example: creating inline files¶

In COPY commands source parameters can be replaced with here-doc indicators. Regular here-doc variable expansion and tab stripping rules apply.

# syntax = docker/dockerfile:1.4
FROM alpine
ARG FOO=bar
COPY <<-eot /app/foo
    hello ${FOO}
eot

# syntax = docker/dockerfile:1.4
FROM alpine
COPY <<-"eot" /app/script.sh
    echo hello ${FOO}
eot
RUN FOO=abc ash /app/script.sh

Built-in build args¶

Name	Type	Description
`BUILDKIT_CACHE_MOUNT_NS`	`string`	Set optional cache ID namespace.
`BUILDKIT_CONTEXT_KEEP_GIT_DIR`	`bool`	Trigger git context to keep the `.git` directory.
`BUILDKIT_MULTI_PLATFORM`	`bool`	Opt into determnistic output regardless of multi-platform output or not.
`BUILDKIT_SANDBOX_HOSTNAME`	`string`	Set the hostname (default `buildkitsandbox`).
`BUILDKIT_SYNTAX`	`string`	Set optional cache ID namespace.
`BUILDKIT_CACHE_MOUNT_NS`	`image`	Set frontend image.

buildctl build \
    --frontend gateway.v0 \
    --opt source=docker/dockerfile \
    --opt context=https://github.com/docker/cli.git \
    --opt build-arg:BUILDKIT_CONTEXT_KEEP_GIT_DIR=1

Value required ↩↩↩

Dockerfile frontend¶

About¶

Build a Dockerfile with an external Dockerfile frontend¶

Using an external Dockerfile frontend¶

Syntax¶

Build mounts RUN --mount=...¶

RUN --mount=type=bind¶

RUN --mount=type=cache¶