Hi Robert,
We are in the process of migrating gitlab.common-lisp.net to the new host, currently gitlab-future.common-lisp.net.
https://gitlab-future.common-lisp.net is up and running and it's been restored from a recent backup of the production host - so if you'd like, you can test the pipeline in question on gitlab-future.common-lisp.net -- it may or may not work completely but at least you can see if it hits a different issue from what you are seeing on the legacy host.
We're hoping to do the cutover to the new gitlab host possibly by the end of the year, and it will be great if we can confirm that the docker pipeline issue you were hitting is indeed not an issue on the upgraded host.