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Make Docker-in-Docker builds faster with Docker layer caching

When using Docker-in-Docker, Docker downloads all layers of your image every time you create a build. Recent versions of Docker (Docker 1.13 and later) can use a pre-existing image as a cache during the docker build step. This significantly accelerates the build process.

How Docker caching works

When running docker build, each command in Dockerfile creates a layer. These layers are retained as a cache and can be reused if there have been no changes. Change in one layer causes the recreation of all subsequent layers.

To specify a tagged image to be used as a cache source for the docker build command, use the --cache-from argument. Multiple images can be specified as a cache source by using multiple --cache-from arguments. Any image that's used with the --cache-from argument must be pulled (using docker pull) before it can be used as a cache source.

Docker caching example

This example .gitlab-ci.yml file shows how to use Docker caching:

default:
  image: docker:20.10.16
  services:
    - docker:20.10.16-dind
  before_script:
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY

variables:
  # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
  DOCKER_HOST: tcp://docker:2376
  DOCKER_TLS_CERTDIR: "/certs"

build:
  stage: build
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker push $CI_REGISTRY_IMAGE:latest

In the script section for the build job:

  1. The first command tries to pull the image from the registry so that it can be used as a cache for the docker build command.
  2. The second command builds a Docker image by using the pulled image as a cache (see the --cache-from $CI_REGISTRY_IMAGE:latest argument) if available, and tags it.
  3. The last two commands push the tagged Docker images to the container registry so that they can also be used as cache for subsequent builds.