Use a uv sync installation and caching in dockerfile (best practice)., instead of outputting requirements.txt

[spwoodcock]

Mentioned in #64

See other repos like drone-tm, field-tm, fair, etc for how to do this.

[Tarekchehahde]

Opened PR: https://github.com/hotosm/website/pull/

Uses uv sync + BuildKit cache mounts instead of exporting requirements.txt.