docker-build:
  # docker-in-docker: the runner uses docker from inside a docker container to execute the CI-jobs
  image: docker:20 # provides the docker toolset (but without an active daemon)

  stage: build

  rules:
    - changes: # only run CI when these files have changed
      - "*.py"
      - "Dockerfile"
      - ".gitlab-ci.yml"

  # services configure images that run during jobs linked to the image (above)
  services:
  - docker:dind # dind build on docker and starts up the dockerdaemon (docker itself doesn't do that), which is needed to call docker build etc.

  before_script:
    # docker login asks for the password to be passed through stdin for security
    - docker login -u $CI_REGISTRY_USER -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY

  script:
    # fetches the latest image from this projects registry to use as cache (not failing if image is not found)
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    # build
    - >
      docker build
      --pull
      --cache-from $CI_REGISTRY_IMAGE:latest
      --label "org.opencontainers.image.title=$CI_PROJECT_TITLE"
      --label "org.opencontainers.image.url=$CI_PROJECT_URL"
      --label "org.opencontainers.image.created=$CI_JOB_STARTED_AT"
      --label "org.opencontainers.image.revision=$CI_COMMIT_SHA"
      --label "org.opencontainers.image.version=$CI_COMMIT_REF_NAME"
      --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
      .
    # tag and push
    - docker tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA $CI_REGISTRY_IMAGE:latest
    - docker push $CI_REGISTRY_IMAGE:latest