Nice tricks! I wasn't aware of the cache mounts, so I was building with UV_NO_CACHE=1. Cache mounts should also come handy when installing OS packages in multi-stage builds.
I still couldn't find the "global pip environment" part, unless by that you meant the "active pip environment", pointed by VIRTUAL_ENV during image building.
Source? That's an option, but it's not even explicitly mentioned in the related documentation [1].
[1] https://docs.astral.sh/uv/guides/integration/docker/