The Automated build feature is not set up properly.

This is missing: https://docs.docker.com/docker-hub/builds/#repository-links

Repository links let you link one Automated Build with another. If one Automated Build gets updated, Docker triggers a build of the other. This makes it easy to ensure that related images are kept in sync. You can link more than one image repository. You only need to link one side of two related builds. Linking both sides causes an endless build loop.

Bonus points for setting up Remote Build triggers. This would allow to recreate the image when a new version of the gem is getting released.