Deploy an app from Gitlab registry to Heroku (.Net Core example). See -> http://blog.thomasmalicet.com/2018/04/deploy-app-from-gitlabs-registry-to.html

.gitlab-ci.yml

image: docker:git
services:
  - docker:dind

variables:
  DOCKER_DRIVER: overlay
  # You can find this on your Gitlab repo page, under "Registry"
  CONTAINER_GITLAB: registry.gitlab.com/<USER OR GROUP NAME>/<REPO NAME>
  # It's just a tag, use whatever you want : back, front, test, staging...
  CONTAINER_TAG: latest
  # Change with the name of your Heroku app
  CONTAINER_HEROKU: registry.heroku.com/<HEROKU APP NAME>/web
  # Can be found on: https://dashboard.heroku.com/account under "API Key"
  HEROKU_API_KEY: <LOOKS LIKE A GUID>
  # Heroku User name for login: usually an e-mail address
  HEROKU_USER: myname@company.com

# Order of tasks
stages:
  - dockerize
  - deploy
  
# Register in your gitlab registry. The environment variables are set automatically, don't change anything.
before_script:
  - docker login -u gitlab-ci-token -p "$CI_BUILD_TOKEN" "$CI_REGISTRY"

dockerize:
  stage: dockerize
  script:
    # Make an image from the current folder and tag it with the registry name and tag you defined
    - docker build -t $CONTAINER_GITLAB:$CONTAINER_TAG .
    # Push to Gitlab
    - docker push $CONTAINER_GITLAB:$CONTAINER_TAG
  only:
    - master

# At this point, you could go to your repo "Registry" page on Gitlab and see your image

deploy_to_heroku:
  stage: deploy
  # On Your CI/CD page on Gitlab, under "Environment", you can add new ones with informations like these and get informations about this deploy task
  environment:
    name: staging
    url: https://<HEROKU APP NAME>.herokuapp.com/
  services:
  - docker:dind
  script:
    # Login to Heroku's registry
    - docker login --username=$HEROKU_USER --password=$HEROKU_API_KEY registry.heroku.com
    # Gets your Gitlab image
    - docker pull $CONTAINER_GITLAB:$CONTAINER_TAG
    # Make it good looking for Heroku
    - docker tag $CONTAINER_GITLAB:$CONTAINER_TAG $CONTAINER_HEROKU
    # Push it to Heroku
    - docker push $CONTAINER_HEROKU
  only:
    - master