Dockerfile: use pinned Python dependencies

[pjonsson]

This keeps the image buildable even
when there are new incompatible
releases available.

Refs davidfrantz/force#403

[pjonsson]

There are multiple ways to pin dependencies in Python, but something like pip-compile is required to pin the transitive dependencies. The reason I used uv for this PR is because it's the least painful way I've found to manage requirements.txt, uv is both fast and tends to do what you expect it to do.

The requirements.txt is automatically generated from the content of pyproject.toml so you can assume requirements.txt reflects that, and focus the review on whether pyproject.toml contains the right dependencies and constraints for them.