Use Docuum to manage Docker image caches on CI runners

h/t https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27332#note_1428874838

https://github.com/stepchowfun/docuum

Docuum performs least recently used (LRU) eviction of Docker images to keep the disk usage below a given threshold. ...

Docuum listens for notifications via docker events to learn when images are used.