How to reduce storage used for Gitlab-integrated Docker Container Registry

I have a free GitLab account. I understand that GitLab will be imposing storage limits on free accounts in the near future.

When I check my storage usage, it says 12.8 GiB “Namespace storage used”. 475.5 KiB of that is used by the one project I currently have (a fork of pdfgrep/pdfgrep); the remaining 12.6 GiB is used by “Gitlab-integrated Docker Container Registry for storing Docker Images”.

How can I reduce this to sensible proportions? I don’t even know what it is. I have never used Docker. I have never created any of my own projects in my GitLab account. I only have a GitLab account so that I can fork other people’s projects and submit trivial fixes to them. How did this storage become so large? How can I prevent it from happening again in the future?

The problem appears to have resolved itself. I had two other forked projects, and since my merge requests from those projects had already been merged, I deleted those forks, hoping that perhaps the unexpected storage usage was somehow connected with those, despite not being indicated as such, leaving just the one 475.5 KiB project. Immediately after deleting those two forks, the “Namespace storage used” reduced only slightly (from 13.2 GiB to 12.8 GiB), but now, 14 hours later, it has reduced all the way to the expected 475.5 KiB and “Gitlab-integrated Docker Container Registry for storing Docker Images” shows 0 bytes used. Perhaps some period cleanup task ran.

I’d still like to understand what happened and how to avoid using unnecessary storage in the future.