I want to try out setting up a slicer-package-manager, and the docker-compose is not working.

This may be a duplicate issue for #79, but it seems that issue is more about including docker-compose in CI.
Also the closed #73 may be related, but probably not, as this is about main.

What seems actually broken is the Dockerfile -- it refers to files MANIFEST.in and setup.cfg which no longer exist. Also, the base docker girder-3.1.0-py3 is a bit ancient.

I'll have some fix suggestions soon.