.gitlab-ci.yml

For WPengine (.gitlab-ci.yml)

before_script:
  - mkdir -p ~/.ssh
  - eval $(ssh-agent -s)
  - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
  - echo "$PRIVATEKEY" | tr -d '\r' | ssh-add - > /dev/null

stages:
  - deploy_staging
  - deploy_master

Deploy Master:
  stage: deploy_master
  script:
    - git remote add production $production-remote-url
    - "git checkout -b master"
    - "git push production master:master -f"
  only:
  - master

Deploy Staging:
  stage: deploy_staging
  script:
    - git remote add staging $staging-remote-url
    - "git checkout -b staging"
    - "git push staging staging:master -f"
  only:
  - staging

Thanks for your sharing. I recently updated a full code of this, but worked and tested. Hope someone found it helpful!

https://gist.github.com/khoipro/4df13fe0231d37a9bd8ae5493c6d7f97