Does Nix do one layer per dependency? Does it run into >=128 layers issues?

In Spack [1] we do one layer per package; it's appealing, but I never checked if besides the layer limit it's actually bad for performance when doing filesystem operations.

[1] https://spack.readthedocs.io/en/latest/containers.html

Yes but then you're committed to using Nix which doesn't work so well the moment you need some software not packaged by Nix.

Want to throw a requirements.txt in there? No no, why would you even ask that? Meanwhile docker says yeah sure just run pip install, why should I care?

Especially if you use nix2container to take control over the layer construction and caching.