Upgrade docker-compose installation of Gitlab

If you are using a standard installation of Gitlab with Docker, it means that you are using the omnibus Gitlab installation. And it's excellent!

What is not that great however is that the upgrade way is not well documented. But it's actually super-easy:

1. Modify your docker-compose.yaml file and specify the new version of Gitlab (and GitLab runner if required). You can find new versions on Docker Hub.
2. Use docker-compose pull to get new versions of GitLab
3. Use docker-compose down to stop GitLab
4. Use docker-compose up -d to start Gitlab again, it will run the upgrade scripts automatically! (it takes a while)

⚠ Warning ️⚠️ You need to follow the upgrade path of Gitlab, which means you cannot go to the last version in one step. You can find some documentation about the upgrade path here. You may also want to use the tool they designed to generate your own upgrade path. https://gitlab-com.gitlab.io/support/toolbox/upgrade-path/

One more thing: you may experiment with issues while upgrading. You can find more information about it by using docker-compose logs -f. And depending on the issue you see, you can find information in the update folder of Gitlab (choose the version you're upgrading to in the tag list)