Containerizing Applications in GitLab CI/CD

Docker Integration in GitLab CI/CD

Building container images inside CI/CD pipelines ensures that your application builds are repeatable, uniform, and deployment-ready. This guide details how to build and store Docker containers using the GitLab Container Registry.

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1. Setting Up Docker-in-Docker (DinD)

To run Docker commands inside a pipeline job, we must use a Docker-capable runner and specify the DinD service:

build_image:
  stage: build
  image: docker:24.0.5
  services:
    - docker:24.0.5-dind
  variables:
    DOCKER_TLS_CERTDIR: "/certs"
  script:
    - docker build -t my-app .

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2. Pushing to GitLab Container Registry

Every GitLab project comes with a built-in container registry. You can authenticate using pre-defined pipeline variables ($CI_REGISTRY_USER, $CI_REGISTRY_PASSWORD, and $CI_REGISTRY):

publish_image:
  stage: deploy
  image: docker:24.0.5
  services:
    - docker:24.0.5-dind
  script:
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
    - docker build -t $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA -t $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker push $CI_REGISTRY_IMAGE:latest

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3. Promoting Images

For release staging, it is common to pull a verified container image tagged with a Git commit SHA, verify it in a smoke-test environment, and promote it to stable or production:

promote_image:
  stage: deploy
  script:
    - docker pull $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA $CI_REGISTRY_IMAGE:stable
    - docker push $CI_REGISTRY_IMAGE:stable

This ensures that only fully tested images are ever deployed to production servers.