Gitlab-CI job to push code to another git forge via SSH

.gitlab-to-github-ci.yml

# the following variables must be defined in "Settings > CI/CD > Variables"
# EXTERNAL_DEPLOY_HOST = github.com
# EXTERNAL_DEPLOY_REPO = [email protected]:USERNAME/REPONAME.git
# SSH_PRIVATE_KEY = content of your SSH private key file /!\ Store as type "File" and not "Variable"

sync-repo:
  stage: deploy
  before_script:
    # fix "Permissions 0666 are too open, private key ignored" error
    - chmod 400 $SSH_PRIVATE_KEY
    # Install ssh-agent if not already installed
    - 'command -v ssh-agent >/dev/null || ( apt-get update -y && apt-get install openssh-client -y )'
    # Run ssh-agent (inside the build environment)
    - eval $(ssh-agent -s)
    # Add the SSH key stored in SSH_PRIVATE_KEY variable to the agent store
    - ssh-add $SSH_PRIVATE_KEY
    # Create the SSH directory and give it the right permissions
    - mkdir -p ~/.ssh
    - chmod 700 ~/.ssh
    # Fix Host key verification failed
    - touch ~/.ssh/known_hosts
    - ssh-keyscan $EXTERNAL_DEPLOY_HOST >> ~/.ssh/known_hosts
    - chmod 644 ~/.ssh/known_hosts
    # git
    - git config --global user.email "[email protected]"
    - git config --global user.name "${CI_PROJECT_TITLE}-bot"
    # gitlab-runner runs on a detached HEAD, checkout current branch
    - git checkout -b $CI_COMMIT_BRANCH
    # set remote push URL
    - git remote set-url --push origin "${EXTERNAL_DEPLOY_REPO}"
  script:
    # push changes to originating branch
    - git push --tags origin ${CI_COMMIT_BRANCH}:${CI_COMMIT_BRANCH}
  environment:
    name: deploy/$CI_COMMIT_REF_SLUG
    action: start
  rules:
    - if: '($CI_PIPELINE_SOURCE == "push" || $CI_PIPELINE_SOURCE == "web") && ($CI_COMMIT_BRANCH == "production"' || $CI_COMMIT_BRANCH == "staging"')