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Building Docker Images on CircleCI 2.0

Overview

For security reasons, the Docker Executor doesn’t allow building Docker images within a job space.

To help users build, run, and publish new images, we’ve introduced a special feature which creates a separate environment for each build. This environment is remote, fully-isolated and has been configured to execute Docker commands.

Configuration

If your build requires docker or docker-compose commands, you’ll need to add a special step into your .circleci/config.yml:

jobs:
  build:
    steps:
      # ... steps for building/testing app ...

      - setup_remote_docker

When setup_remote_docker executes, a remote environment will be created, and your current primary container will be configured to use it. Then, any docker-related commands you use will be safely executed in this new environment.

Example

Here’s an example where we build and push a Docker image for our demo docker project:

version: 2
jobs:
  build:
    docker:
      - image: golang:1.6.4   # (1)
    working_directory: /go/src/github.com/circleci/cci-demo-docker
    steps:
      - checkout
      # ... steps for building/testing app ...

      - setup_remote_docker   # (2)

      # use a primary image that already has Docker (recommended)
      # or install it during a build like we do here
      - run:
          name: Install Docker client
          command: |
            set -x
            VER="17.03.0-ce"
            curl -L -o /tmp/docker-$VER.tgz https://get.docker.com/builds/Linux/x86_64/docker-$VER.tgz
            tar -xz -C /tmp -f /tmp/docker-$VER.tgz
            mv /tmp/docker/* /usr/bin

      # build and push Docker image
      - run: |
          TAG=0.1.$CIRCLE_BUILD_NUM
          docker build -t circleci/cci-demo-docker:$TAG .      # (3)
          docker login -u $DOCKER_USER -p $DOCKER_PASS         # (4)
          docker push circleci/cci-demo-docker:$TAG

Let’s break down what’s happening during this build’s execution:

  1. All commands are executed in the primary container.
  2. Once setup_remote_docker is called, a new remote environment is created, and your primary container is configured to use it.
  3. All docker-related commands are also executed in your primary container, but building/pushing images and running containers happens in the remote Docker Engine.
  4. We use project environment variables to store credentials for Docker Hub.

Separation of Environments

Since the job space and remote docker are separated environments, there’s one caveat: containers running in your job space can’t directly communicate with containers running in remote docker.

Accessing Services

It’s impossible to start a service in remote docker and ping it directly from a primary container (and vice versa). To solve that, you’ll need to interact with a service from remote docker, as well as through the same container:

# start service and check that it’s running
- run: |
    docker run -d --name my-app my-app
    docker exec my-app curl --retry 10 --retry-connrefused http://localhost:8080

A different way to do this is to use another container running in the same network as the target container:

- run: |
    docker run -d --name my-app my-app
    docker run --network container:my-app appropriate/curl --retry 10 --retry-connrefused http://localhost:8080

Mounting Folders

It’s not possible to mount a folder from the build container into an isolated Docker container (and vice versa).

If you have any questions, head over to our community forum for support from us and other users.