Gitlab Docker container registry auto-rebuild


We are told:

With the help of GitLab CI you can automatically rebuild images that inherit from yours, allowing you to easily deliver fixes and new features to a base image used by your teams.

How do we get images that inherit from a base image to auto-rebuild when the base image is updated? This does not seem to happen automatically. In my case both the baseline image and the inherited image are in the Gitlab Container Registry, under different projects.