Add a Dockerfile

[signebedi]

This should be more straightforward since there are fewer requirements than in libreForms-flask. The assumption will be in favor of SQLite... but I think we will need to allow users to point to a different database. Also, what to do about MongoDB...? Ideally, we just need a python base image... but, if we can't handle MongoDB without forcing users to handle it separately or use docker compose, then that probably is not workable.