I wanted to spin up a mirror locally to do simple caching for docker builds but the tooling was lacking, there was a way to do a direct mirror of pypi locally but no other way of adding custom indices

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.)

It might honestly be easier and cheaper for those CI providers to just pay PyPI's bandwidth costs.