ci: add optional mr build image

  image: docker:latest

  rules:

    - if: $CI_PIPELINE_SOURCE != "merge_request_event" && $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

      when: on_success

      variables:

        CONTAINER_TAG: latest

    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

      when: manual

      variables:

        CONTAINER_TAG: $CI_COMMIT_REF_SLUG

    - when: never

  before_script:

    - echo $CI_REGISTRY_PASSWORD | docker login $CI_REGISTRY -u $CI_REGISTRY_USER --password-stdin

  variables:

    CONTAINER_IMAGE_NAME: "$CI_REGISTRY_IMAGE:$CONTAINER_TAG"

  script:

    - docker build --tag $CI_REGISTRY_IMAGE .

    - docker push $CI_REGISTRY_IMAGE

    - docker build --tag $CONTAINER_IMAGE_NAME .

    - docker push $CONTAINER_IMAGE_NAME