How to run experimental Docker features on CircleCI

When a build is started on CircleCI one of the first things that is done is:


Setup a remote Docker engine

Specified reusable docker engine, but build has not been whitelisted.
Contact CircleCI to be whitelisted
Allocating a remote Docker Engine
...
Remote Docker engine created. Using VM 'prealloc-wrjtu1qd-1491949826270'
Created container accessible with:
  DOCKER_TLS_VERIFY=1
  DOCKER_HOST=tcp://:2376
  DOCKER_CERT_PATH=/tmp/docker-certs615987123
  DOCKER_MACHINE_NAME=51123

Subsequently when docker build --squash -t imagename . is run on CircleCI it results in:

Error response from daemon: squash is only supported with experimental mode
Exited with code 1

Discussion

It works to use experimental features locally after changing the /etc/docker/daemon.json as follows:

{
    "experimental": true
}

and restarting the docker systemctl service, but how to do that on CircleCI? It seems impossible as the build output indicates that a connection is made to a remote docker that reside on the CircleCI systems.

CircleCI now allows you to use a machine executor which provisions a separate VM for you, with Ubuntu 14.04 and Docker version 17.06.0-ce installed. This allows you to turn on experimental features for the Docker daemon.

You need to use the machine key instead of the docker key, to run your job in a separate virtual machine instead of just a Docker container.

You can only choose 2 images for the machine:

• circleci/classic:latest: Ubuntu 14.04 with Docker 17.03.0-ce, or

• circleci/classic:edge: Ubuntu 14.04 with Docker 17.06.0-ce - the one with experimental features.

You'll also need to install dependencies on the machine yourself, as it is quite bare. For example, if you need PHP for your tests, you'll need to run sudo apt-get install -y php5.

Here is a sample .circleci/config.yml that builds a Docker image using experimental feature docker build --squash:


.circleci/config.yml

version: 2
jobs:
  build:
    # Run in a separate virtual machine instead of a Docker container.
    machine:
      enabled: true
      # Use Ubuntu 14.04 with bleeding edge Docker daemon 17.06.0-ce.
      image: circleci/classic:edge
    steps:
      - checkout
      - run:
          command: |
            # Restart Docker with experimental features on.
            sudo sh -c 'echo '\''DOCKER_OPTS="--experimental=true"'\'' >> /etc/default/docker'
            sudo service docker restart

            # Install dependencies for tests etc.
            sudo apt-get update
            sudo apt-get install -y php5

            # Build image with experimental feature --squash.
            docker build --squash -t myuser/myimage .

            # Login and push Docker image to registry.
            docker login -u $DOCKER_USER -p $DOCKER_PASS
            docker push myuser/myimage

StackExchange DevOps Q#849, answer score: 8