Dockerfile frontend syntaxes

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

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.

Using 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.

Linked copies `COPY --link`, `ADD --link`

To use this flag set Dockerfile version to at least 1.4.

# syntax=docker/dockerfile:1.4

Enabling this flag in COPY or ADD commands allows you to copy files with enhanced semantics where your files remain independent on their own layer and don't get invalidated when commands on previous layers are changed.

When --link is used your source files are copied into an empty destination directory. That directory is turned into a layer that is linked on top of your previous state.

# syntax=docker/dockerfile:1.4
FROM alpine
COPY --link /foo /bar

Is equivalent of doing two builds:

FROM alpine

and

FROM scratch
COPY /foo /bar

and merging all the layers of both images together.

Benefits of using `--link`

Using --link allows to reuse already built layers in subsequent builds with --cache-from even if the previous layers have changed. This is especially important for multi-stage builds where a COPY --from statement would previously get invalidated if any previous commands in the same stage changed, causing the need to rebuild the intermediate stages again. With --link the layer the previous build generated is reused and merged on top of the new layers. This also means you can easily rebase your images when the base images receive updates, without having to execute the whole build again. In backends that support it, BuildKit can do this rebase action without the need to push or pull any layers between the client and the registry. BuildKit will detect this case and only create new image manifest that contains the new layers and old layers in correct order.

The same behavior where BuildKit can avoid pulling down the base image can also happen when using --link and no other commands that would require access to the files in the base image. In that case BuildKit will only build the layers for the COPY commands and push them to the registry directly on top of the layers of the base image.

Incompatibilities with `--link=false`

When using --link the COPY/ADD commands are not allowed to read any files from the previous state. This means that if in previous state the destination directory was a path that contained a symlink, COPY/ADD can not follow it. In the final image the destination path created with --link will always be a path containing only directories.

If you don't rely on the behavior of following symlinks in the destination path, using --link is always recommended. The performance of --link is equivalent or better than the default behavior and it creates much better conditions for cache reuse.

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` (the default mount type)

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

Option	Description
`target` (required)	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` (required)	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` (required)	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=none|host|default`

# syntax=docker/dockerfile:1.3

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

The allowed values are:

none - The command is run with no network access (lo is still available, but is isolated to this process)
host - The command is run in the host's network environment (similar to docker build --network=host, but on a per-instruction basis)
default - Equivalent to not supplying a flag at all, the command is run in the default network for the build

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).

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.

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

Security context `RUN --security=insecure|sandbox`

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

# syntax=docker/dockerfile:1.3-labs

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. 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).

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

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

Built-in build args

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_INLINE_BUILDINFO_ATTRS=<bool>¹ inline build info attributes in image config or not
BUILDKIT_INLINE_CACHE=<bool>¹ inline cache metadata to image config or not
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=<image> set frontend image

¹ For Docker-integrated BuildKit (DOCKER_BUILDKIT=1 docker build) and docker buildx

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syntax.md

syntax.md

Dockerfile frontend syntaxes

Note for Docker users

Using external Dockerfile frontend

Linked copies `COPY --link`, `ADD --link`

Benefits of using `--link`

Incompatibilities with `--link=false`

Build Mounts `RUN --mount=...`

`RUN --mount=type=bind` (the default mount type)

`RUN --mount=type=cache`

Example: cache Go packages

Example: cache apt packages

`RUN --mount=type=tmpfs`

`RUN --mount=type=secret`

Example: access to S3

`RUN --mount=type=ssh`

Example: access to Gitlab

Network modes `RUN --network=none|host|default`

Example: isolating external effects

Here-Documents

Example: running a multi-line script

Example: creating inline files

Security context `RUN --security=insecure|sandbox`

Example: check entitlements

Built-in build args

Files

syntax.md

Latest commit

History

syntax.md

File metadata and controls

Dockerfile frontend syntaxes

Note for Docker users

Using external Dockerfile frontend

Linked copies COPY --link, ADD --link

Benefits of using --link

Incompatibilities with --link=false

Build Mounts RUN --mount=...

RUN --mount=type=bind (the default mount type)

RUN --mount=type=cache

Example: cache Go packages

Example: cache apt packages

RUN --mount=type=tmpfs

RUN --mount=type=secret

Example: access to S3

RUN --mount=type=ssh

Example: access to Gitlab

Network modes RUN --network=none|host|default

Example: isolating external effects

Here-Documents

Example: running a multi-line script

Example: creating inline files

Security context RUN --security=insecure|sandbox

Example: check entitlements

Built-in build args

Linked copies `COPY --link`, `ADD --link`

Benefits of using `--link`

Incompatibilities with `--link=false`

Build Mounts `RUN --mount=...`

`RUN --mount=type=bind` (the default mount type)

`RUN --mount=type=cache`

`RUN --mount=type=tmpfs`

`RUN --mount=type=secret`

`RUN --mount=type=ssh`

Network modes `RUN --network=none|host|default`

Security context `RUN --security=insecure|sandbox`