GitLab CI - build and push docker image to GitLab registry

.gitlab-ci.yml

stages:
  - pack

### Pack ######################################################################

docker:
  stage: pack
  environment: production
  image: docker:18
  script:
    # replace '/' with '-' to make it a valid docker tag
    - IMAGE_TAG=${CI_COMMIT_REF_NAME//\//-} 
    - | # remove leading 'v' for version tags
      if [[ "$CI_COMMIT_TAG" = v[0-9]* ]]; then 
        IMAGE_TAG=${IMAGE_TAG#v}
      fi 
    - echo -n $CI_REGISTRY_PASSWORD | docker login --username $CI_REGISTRY_USER --password-stdin $CI_REGISTRY
    - docker build . -t $CI_REGISTRY_IMAGE:$IMAGE_TAG
    - | # push docker image for version tags
      if [[ "$CI_COMMIT_TAG" = v[0-9]* ]]; then
        docker push $CI_REGISTRY_IMAGE:$IMAGE_TAG
      fi