Specifying Container Images

This document describes images and containers in the following sections:

• Overview
  • Using Docker
  • Using Machine
• Using Multiple Docker Images
• Docker Benefits and Limitations
• Docker Image Best Practices

Overview

This version of CircleCI enables you to use Docker images or to use a dedicated VM image.

There are tradeoffs to docker versus machine, as follows:

⁽¹⁾ Requires using Remote Docker.

Using Docker

The docker key defines Docker as the underlying technology to run your jobs using Docker Containers. Containers are an instance of the Docker Image you specify and the first image listed in your configuration is the primary container image in which all steps run.

Docker increases performance by building only what is required for your application. Specify a Docker image in your .circleci/config.yml file that will generate the primary container where all steps run:

jobs:
  build:
    docker:
      - image: buildpack-deps:trusty

In this example, all steps run in the container created by the first image listed under the build job. To make the transition easy, CircleCI maintains convenience images on Docker Hub for popular languages. See Using Pre-Built CircleCI Docker Images for the complete list of names and tags. If you need a Docker image that installs Docker and has Git, consider using docker:stable-git, which is an offical Docker image.

Using Machine

The machine option will run your jobs in a dedicated, ephemeral Virtual Machine (VM). Note: Use of machine may require additional fees in a future pricing update.

To use the machine executor with the default machine image, set the machine key to true in .circleci/config.yml:

jobs:
  build:
    machine: true

Using the machine executor enables your application with full access to OS resources and provides you with full control over the job environment, if for example, you need to use ping or to modify system with sysctl commands. In addition, it enables your repo to build a docker image without additional downloads for languages like Ruby and PHP.

This example specifies a particular image for the machine executor:

jobs:
  build:
    machine:
      image: circleci/classic:201708-01

Following are the available machine images:

• circleci/classic:latest is the default image. Changes to this will be announced at least one week before they go live.
• circleci/classic:edge receives the latest updates and will be upgraded at short notice.
• circleci/classic:[year-month] This lets you pin the image version to prevent breaking changes. Refer to Writing Jobs with Steps for versions.

The images have common language tools preinstalled. Refer to the specification script for the VM for more information about additional tools.

Using Multiple Docker Images

It is possible to specify multiple images for your job. Specify multiple images if, for example, you need to use a database for your tests or for some other required service. In a multi-image configuration job, all steps are executed in the container created by the first image listed. All containers run in a common network and every exposed port will be available on localhost from a primary container.

jobs:
  build:
    docker:
    # Primary container image where all steps run.
     - image: buildpack-deps:trusty
    # Secondary container image on common network. 
     - image: mongo:2.6.8
       command: [mongod, --smallfiles]

    working_directory: ~/

    steps:
      # command will execute in trusty container
      # and can access mongo on localhost
      - run: sleep 5 && nc -vz localhost 27017

Docker Images may be specified in three ways, by the image name and version tag on Docker Hub or by using the URL to an image in a registry:

Public Convenience Images on Docker Hub

• name:tag
  • circleci/node:7.10-browsers
• name@digest
  • redis@sha256:34057dd7e135ca41...

Public Images on Docker Hub

• name:tag
  • alpine:3.4
• name@digest
  • redis@sha256:54057dd7e125ca41...

Public Docker Registries

• image_full_url:tag
  • gcr.io/google-containers/busybox:1.24
• image_full_url@digest
  • gcr.io/google-containers/busybox@sha256:4bdd623e848417d9612...

Nearly all of the public images on Docker Hub and Docker Registry are supported by default when you specify the docker: key in your config.yml file. If you want to work with private images/registries, please refer to Using Private Images.

Docker Benefits and Limitations

Docker also has built-in image caching and enables you to build, run, and publish Docker images via Remote Docker. Consider the requirements of your application as well. If the following are true for your application, Docker may be the right choice:

• Your application is self-sufficient
• Your application requires additional services to be tested
• Your application is distributed as a Docker Image (requires using Remote Docker)
• You want to use docker-compose (requires using Remote Docker)

Choosing Docker limits your runs to what is possible from within a Docker container (including our Remote Docker feature). For instance, if you require low-level access to the network or need to mount external volumes consider using machine.

Docker Image Best Practices

• Avoid using mutable tags like latest or 1 as the image version in your config.yml file. Mutable tags often lead to unexpected changes in your job environment. CircleCI cannot guarantee that mutable tags will return an up-to-date version of an image. You could specify alpine:latest and actually get a stale cache from a month ago. Instead, we recommend using precise image versions or digests, like redis:3.2.7 or redis@sha256:95f0c9434f37db0a4f... as shown in the examples.

• If you experience increases in your run times due to installing additional tools during execution, it is best practice to use the Building Custom Docker Images Documentation to create a custom image with tools that are pre-loaded in the container to meet the job requirements.

More details on the Docker Executor are available here.