Allow users to set the UID for the hackmd user

[m0wer]

It would be nice to be able to set the uid and gid for the hackmd user inside the container.

[m0wer]

For example, adding an entrypoint as the one used in gitea will easily fix this issue:
https://github.com/go-gitea/gitea/blob/master/docker/usr/bin/entrypoint

[m0wer]

ping