Add documentation for running as unprivileged user

[returnDanilo]

Hey! I took the initiative of writing a bit of docs for the things I had to figure out myself. Plus, it was asked in #10047.

I think the docs could use a paragraph explaining the case of using cache mounts. But I've encountered the limits of my understanding of docker and uv.

Here's the problem I found when trying to document it:

--mount=type=cache leaves a root-owned directory after the RUN instruction has run; even when using the uid=.../gid=... options. I don't know if this is intended behavior or a bug.

This makes it so that you need to either (1) rmdir it before allowing the command in CMD ["uv", "run", "foobar.py"] to run, or (2) reset UV_CACHE_DIR to point to somewhere else.

Regardless of 1 or 2, the cache at "build" time becomes useless for the program at run time, and I don't know how important that is.