$ docker build -f Mockerfile.yaml .

Writing a YAML Docker-compatible alternative to the Dockerfile


In this blog post, I'll show you how to write your own Dockerfile syntax that works out of the box with any existing Docker installation.

If you want to see it in action right away, here's a YAML file that is used in place of a Dockerfile.

curl https://raw.githubusercontent.com/r2d4/mockerfile/master/Mockerfile.yaml | DOCKER_BUILDKIT=1 docker build -f - .

The sample code for this post can be found on GitHub.

Background

Buildkit is a tool that can convert code to docker images. It's already integrated in Docker versions  18.09  and above.

Buildkit works by mapping a human-readable frontend (e.g. Dockerfile) to a set of Ops (ExecOp, CacheOp, SecretOp, CopyOp, SourceOp, etc.), collectively called low-level builders (LLB).

That LLB is then executed by either a runc or containerd worker and produces a docker image.

Design

Our demo frontend is going to be called Mockerfile. It's going to be a YAML based syntactic sugar for building ubuntu-based images. It will contain two keys: package, which is some automation around apt-get, and external, which will fetch external dependencies concurrently.

#syntax=r2d4/mocker
apiVersion: v1alpha1
images:
- name: demo
  from: ubuntu:16.04
  package:
    repo: 
    - deb [arch=amd64] http://storage.googleapis.com/bazel-apt stable jdk1.8
    - deb [arch=amd64] https://download.docker.com/linux/ubuntu xenial edge
    gpg: 
    - https://bazel.build/bazel-release.pub.gpg
    - https://download.docker.com/linux/ubuntu/gpg
    install:
    - bazel
    - python-dev
    - ca-certificates
    - curl
    - build-essential
    - git
    - gcc
    - python-setuptools
    - lsb-release
    - software-properties-common
    - docker-ce=17.12.0~ce-0~ubuntu
  external:
  - src: https://storage.googleapis.com/kubernetes-release/release/v1.10.0/bin/linux/amd64/kubectl
    dst: /usr/local/bin/kubectl

  - src: https://github.com/kubernetes-sigs/kustomize/releases/download/v1.0.8/kustomize_1.0.8_linux_amd64
    dst: /usr/local/bin/kustomize
    sha256: b5066f7250beb023a3eb7511c5699be4dbff57637ac4a78ce63bde6e66c26ac4

  - src: https://storage.googleapis.com/kubernetes-helm/helm-v2.10.0-linux-amd64.tar.gz
    dst: /tmp/helm
    install:
    - install /tmp/helm/linux-amd64/helm /usr/local/bin/helm

  - src: https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-sdk-217.0.0-linux-x86_64.tar.gz
    dst: /tmp

Code Walk-through

High level steps

  1. Write a conversion function from your configuration file format to LLB
  2. Write a build function that handles some extra tasks such as mounting the configuration file, secrets, or context.
  3. Use that build function in the frontend gRPC gateway
  4. Publish as a docker image
  5. Add the #syntax=yourregistry/yourimage directive to your top of your config file and set DOCKER_BUILDKIT=1 to build with any Docker installation.

Writing the Conversion Function

Here is my conversion function for Mockerfile. It takes my configuration struct and returns DAG called llb.State.

Some interesting observations:

  • You can start as many different concurrent paths as you want with llb.Image (Similar to a FROM instruction), but those paths must be merged into a final image.
  • Merging is done with a copy helper function, which takes two llb.State, mounts src to dst, and copies the file over, producing a single llb.State. (Similar to a COPY --from multistage build)

The external files are downloaded in separate alpine images, and then use the copy helper to move them into the final image. It uses a small script to verify the checksums of the downloaded binaries s = s.Run(shf("echo \"%s %s\" | sha256sum -c -", e.Sha256, downloadDst)).Root(). If the checksum does not match, the command fails, and the image build stops.

Writing the Build Function

Steps of the build function

  1. Get the Mockerfile/Dockerfile config and build context
  2. Convert config to LLB
  3. Solve the LLB
  4. Package the image and metadata

The configuration file itself must be mounted into the build contianer, for which we use llb.Local. You can see this in action here. Mounting a build context would be done in a similar way.

Creating the gRPC gateway

We reuse the grpc client here. As long as your build function fits the interface type BuildFunc func(context.Context, Client) (*Result, error), things will work as expected.

Publish the image

Our image is quite simple, using the built binary as the entrypoint. The binary runs the grpc gateway we created in the last step. Here is an example.

Using it

  • Add # syntax=yourregistry/yourimage to the top of your configuration file. Buildkit looks for that, and will pull and use that image as the solver.
  • Add DOCKER_BUILDKIT=1 to your docker build command to enable the buildkit backend.
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