[zahlman]

Pip already does its own caching, but it's maddeningly difficult to even locate and extract files, let alone set up anything usable. It's also needlessly difficult to make Pip just use such a cache directly - for example, if you haven't pinned a version, it will automatically check PyPI to figure out the latest version even if you have cached wheels already.

I don't know (I lack the experience), but I assume that container systems get in the way of Pip finding its normal cache, too. (If they're emulating a filesystem or something, then the cache is in the wrong filesystem unless you reuse the container.)