don't require SUPERUSER to run migrations

[CobaltCause]

Fixes #2863.

The first commit fixes the problem, the second one prevents it from happening again.

The second commit is there to convey the idea but obviously it shouldn't be implemented this way since there's an existing docker-compose.yml in the repository. If you can point me to the relevant places this sort of change needs to actually happen, I'll amend that second commit.

Would probably also be wise to test this with more data, I used a completely different hack to work around this for my instance.

[Nutomic]

We normally use the system postgres, its unlikely that we would remember to test again for this rare problem with another postgres. I also dont want to pollute the repo root with more files.

Its better if you can apply this init step to the postgres used by ci, which is defined at the bottom of .woodpecker.yml.

[CobaltCause]

Last force pushes were to rebase onto main and because I forgot to remove the old environment variables for docker/docker-compose.yml which are now controlled by docker/init-user-db.sh. I think this should be ready to go now.