Dockerfile: use `uv install python`

[peteski22]

I agree it would be cleaner to use that, but it seems at present not all architectures work with uv install python (I run into issues on Apple silicon building for local Docker):

astral-sh/python-build-standalone#569 (comment)

I'll leave it as-is for now, and we can review it again if that's OK?

Originally posted by @peteski22 in #94 (comment)