Build Enhancements for Docker

Estimated reading time: 6 minutes

Docker Build is one of the most used features of the Docker Engine - users ranging from developers, build teams, and release teams all use Docker Build.

Docker Build enhancements for 18.09 release introduces a much-needed overhaul of the build architecture. By integrating BuildKit, users should see an improvement on performance, storage management, feature functionality, and security.

  • Docker images created with buildkit can be pushed to Docker Hub and DTR just like Docker images created with legacy build
  • the Dockerfile format that works on legacy build will also work with buildkit builds
  • The new --secret command line option allows the user to pass secret information for building new images with a specified Dockerfile

For more information on build options, see the reference guide on the command line build options.


  • System requirements are docker-ce x86_64, ppc64le, s390x, aarch64, armhf; or docker-ee x86_64 only
  • Network connection required for downloading images of custom frontends


  • BuildKit mode is incompatible with UCP and Swarm Classic
  • Only supported on Linux

To enable buildkit builds

Easiest way from a fresh install of docker is to set the DOCKER_BUILDKIT=1 environment variable when invoking the docker build command, such as:

$ DOCKER_BUILDKIT=1 docker build .

To enable docker buildkit by default, set daemon configuration in /etc/docker/daemon.json feature to true and restart the daemon:

{ "features": { "buildkit": true } }

New Docker Build command line build output

New docker build BuildKit TTY output (default):

$ docker build . 
[+] Building 70.9s (34/59)                                                      
 => [runc 1/4] COPY hack/dockerfile/install/ ./       14.0s
 => [frozen-images 3/4] RUN / /build  buildpa  24.9s
 => [containerd 4/5] RUN PREFIX=/build/ ./ containerd           37.1s
 => [tini 2/5] COPY hack/dockerfile/install/ ./        4.9s
 => [vndr 2/4] COPY hack/dockerfile/install/vndr.installer ./              1.6s
 => [dockercli 2/4] COPY hack/dockerfile/install/dockercli.installer ./    5.9s
 => [proxy 2/4] COPY hack/dockerfile/install/proxy.installer ./           15.7s
 => [tomlv 2/4] COPY hack/dockerfile/install/tomlv.installer ./           12.4s
 => [gometalinter 2/4] COPY hack/dockerfile/install/gometalinter.install  25.5s
 => [vndr 3/4] RUN PREFIX=/build/ ./ vndr                       33.2s
 => [tini 3/5] COPY hack/dockerfile/install/tini.installer ./              6.1s
 => [dockercli 3/4] RUN PREFIX=/build/ ./ dockercli             18.0s
 => [runc 2/4] COPY hack/dockerfile/install/runc.installer ./              2.4s
 => [tini 4/5] RUN PREFIX=/build/ ./ tini                       11.6s
 => [runc 3/4] RUN PREFIX=/build/ ./ runc                       23.4s
 => [tomlv 3/4] RUN PREFIX=/build/ ./ tomlv                      9.7s
 => [proxy 3/4] RUN PREFIX=/build/ ./ proxy                     14.6s
 => [dev 2/23] RUN useradd --create-home --gid docker unprivilegeduser     5.1s
 => [gometalinter 3/4] RUN PREFIX=/build/ ./ gometalinter        9.4s
 => [dev 3/23] RUN ln -sfv /go/src/ ~/.ba  4.3s
 => [dev 4/23] RUN echo source /usr/share/bash-completion/bash_completion  2.5s
 => [dev 5/23] RUN ln -s /usr/local/completion/bash/docker /etc/bash_comp  2.1s

New docker build BuildKit plain output:

$ docker build --progress=plain . 

#1 [internal] load .dockerignore
#1       digest: sha256:d0b5f1b2d994bfdacee98198b07119b61cf2442e548a41cf4cd6d0471a627414
#1         name: "[internal] load .dockerignore"
#1      started: 2018-08-31 19:07:09.246319297 +0000 UTC
#1    completed: 2018-08-31 19:07:09.246386115 +0000 UTC
#1     duration: 66.818µs
#1      started: 2018-08-31 19:07:09.246547272 +0000 UTC
#1    completed: 2018-08-31 19:07:09.260979324 +0000 UTC
#1     duration: 14.432052ms
#1 transferring context: 142B done

#2 [internal] load Dockerfile
#2       digest: sha256:2f10ef7338b6eebaf1b072752d0d936c3d38c4383476a3985824ff70398569fa
#2         name: "[internal] load Dockerfile"
#2      started: 2018-08-31 19:07:09.246331352 +0000 UTC
#2    completed: 2018-08-31 19:07:09.246386021 +0000 UTC
#2     duration: 54.669µs
#2      started: 2018-08-31 19:07:09.246720773 +0000 UTC
#2    completed: 2018-08-31 19:07:09.270231987 +0000 UTC
#2     duration: 23.511214ms
#2 transferring dockerfile: 9.26kB done

Overriding default frontends

The new syntax features in Dockerfile are available if you override the default frontend. To override the default frontend, set the first line of the Dockerfile as a comment with a specific frontend image:

# syntax = <frontend image>, e.g. # syntax = docker/dockerfile:1.0-experimental

New Docker Build secret information

The new --secret flag for docker build allows the user to pass secret information to be used in the Dockerfile for building docker images in a safe way that will not end up stored in the final image.

id is the identifier to pass into the docker build --secret. This identifier is associated with the RUN --mount identifier to use in the Dockerfile. Docker does not use the filename of where the secret is kept outside of the Dockerfile, since this may be sensitive information.

dst renames the secret file to a specific file in the Dockerfile RUN command to use.

For example, with a secret piece of information stored in a text file:

$ echo 'WARMACHINEROX' > mysecret.txt

And with a Dockerfile that specifies use of a buildkit frontend docker/dockerfile:1.0-experimental, the secret can be accessed.

For example:

# syntax = docker/dockerfile:1.0-experimental
FROM alpine
RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret # shows secret from default secret location
RUN --mount=type=secret,id=mysecret,dst=/foobar cat /foobar # shows secret from custom secret location

This Dockerfile is only to demonstrate that the secret can be accessed. As you can see the secret printed in the build output. The final image built will not have the secret file:

$ docker build --no-cache --progress=plain --secret id=mysecret,src=mysecret.txt .
#8 [2/3] RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret
#8       digest: sha256:5d8cbaeb66183993700828632bfbde246cae8feded11aad40e524f54ce7438d6
#8         name: "[2/3] RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret"
#8      started: 2018-08-31 21:03:30.703550864 +0000 UTC
#8    completed: 2018-08-31 21:03:32.051053831 +0000 UTC
#8     duration: 1.347502967s

#9 [3/3] RUN --mount=type=secret,id=mysecret,dst=/foobar cat /foobar
#9       digest: sha256:6c7ebda4599ec6acb40358017e51ccb4c5471dc434573b9b7188143757459efa
#9         name: "[3/3] RUN --mount=type=secret,id=mysecret,dst=/foobar cat /foobar"
#9      started: 2018-08-31 21:03:32.052880985 +0000 UTC
#9    completed: 2018-08-31 21:03:33.523282118 +0000 UTC
#9     duration: 1.470401133s

Using SSH to access private data in builds

Acknowledgment: Please see Build secrets and SSH forwarding in Docker 18.09 for more information and examples.

The docker build has a --ssh option to allow the Docker Engine to forward SSH agent connections. For more information on SSH agent, see the OpenSSH man page.

Only the commands in the Dockerfile that have explicitly requested the SSH access by defining type=ssh mount have access to SSH agent connections. The other commands have no knowledge of any SSH agent being available.

To request SSH access for a RUN command in the Dockerfile, define a mount with type ssh. This will set up the SSH_AUTH_SOCK environment variable to make programs relying on SSH automatically use that socket.

Here is an example Dockerfile using SSH in the container:

# syntax=docker/dockerfile:experimental
FROM alpine

# Install ssh client and git
RUN apk add --no-cache openssh-client git

# Download public key for
RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan >> ~/.ssh/known_hosts

# Clone private repository
RUN --mount=type=ssh git clone myproject

Once the Dockerfile is created, use the --ssh option for connectivity with the SSH agent.

$ docker build --ssh default .
build, security, engine, secret, buildkit