Add Docker instructions

[GeorchW]

Hey, I just recently heard about this project, and I love it! I was wondering how to create docker containers and read through the issue (#241) and discussion (#239). I tried out what works and what seems to be clean to me and wrote it down.

If I understood it correctly, then web apps should (usually) be virtual projects, since they (usually) don't produce a package, so the first part of the guide is a recommendation on how to deal with this case. IMHO "virtual project" isn't the most intuitive name for "doesn't produce a package", and while the documentation on it is alright, it's not very discoverable to me -- I only know about them since I clicked through all the links in the docs. But maybe it's fine to refer to them in the Docker section.

I've refrained from including any sed /^-e/d requirements.lock hacking; I think that would be quite brittle.

I've read somewhere that the lockfile format is not final yet. Is it planned to keep it compatible to requirements.txt? That would influence the guide a lot.

Closes #241

[GeorchW]

(forgot to add a file initially. force-pushed it now)

[bluss]

we have to imagine lockfile outcomes are influenced by what comes out of the python project's own discussions on them https://discuss.python.org/t/lock-files-again-but-this-time-w-sdists/

[mitsuhiko]

I think it's a good start.

[Lxstr]

Maybe needs another issue but I am seeing the following line in my requirements.lock:

The following packages are considered to be unsafe in a requirements file:
setuptools==69.2.0

...despite not needing it and setting:

[tool.rye]
virtual = true

Should I remove this with sed before docker build or is there another way?