Some SageMath development is also happening on GitLab.
We run a read-only mirror of the official branches , a read-only mirror of the branches that people work on on trac , and a collaborative workspace (contact any of the developers there if you want to get access.)
Currently, images for https://hub.docker.com/repository/docker/sagemath/sagemath are not built automatically anymore. The main issue seems to be that there are no dedicated runners registered there anymore.
There are likely other problems since the GitLab Pipeline has not been running for ages now. There does not seem to be a real interest in using GitLab for SageMath development.