image: docker:18

stages:
  - build
  - push

before_script:
  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY

Build:
  stage: build
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    - >
      docker build
      --pull
      --build-arg GITLAB_USER=gitlab-ci-token
      --build-arg GITLAB_PASS=$CI_JOB_TOKEN
      --cache-from $CI_REGISTRY_IMAGE:latest
      --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
      .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA

Push latest:
  variables:
    # We are just playing with Docker here. 
    # We do not need GitLab to clone the source code.
    GIT_STRATEGY: none
  stage: push
  only:
    # Only "master" should be tagged "latest"
    - master
  script:
    # Because we have no guarantee that this job will be picked up by the same runner 
    # that built the image in the previous step, we pull it again locally
    - docker pull $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    # Then we tag it "latest"
    - docker tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA $CI_REGISTRY_IMAGE:latest
    # Annnd we push it.
    - docker push $CI_REGISTRY_IMAGE:latest



# Finally, the goal here is to Docker tag any Git tag
# GitLab will start a new pipeline everytime a Git tag is created, which is pretty awesome
Push tag:
  variables:
    # Again, we do not need the source code here. Just playing with Docker.
    GIT_STRATEGY: none
  stage: push
  only:
    # We want this job to be run on tags only.
    - tags
  script:
    - docker pull $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME