Deploy multiple apps to Heroku from one Repository

.gitlab-ci.yml

stages:
  - deploy

deploy:frontend:
  script:
    - git remote add heroku-frontend https://heroku:$HEROKU_API_KEY@git.heroku.com/frontend-app.git
    - git push --force heroku-frontend `git subtree split --prefix frontend-folder HEAD`:master
  only:
    - master
  stage: deploy

deploy:api:
  script:
    - git remote add heroku-api https://heroku:$HEROKU_API_KEY@git.heroku.com/backend-app.git
    - git push --force heroku-api `git subtree split --prefix backend-folder HEAD`:master
  only:
    - master
  stage: deploy