> running arbitrary commands to invoke language-specific package managers.

This is exactly what we do in Nix. You see this everywhere in nixpkgs.

What sets apart Nix from docker is not that it works well at a finer granularity, i.e. source-file-level, but that it has real hermeticity and thus reliable caching. That is, we also run arbitrary commands, but they don't get to talk to the internet and thus don't get to e.g. `apt update`.

In a Dockerfile, you can `apt update` all you want, and this makes the build layer cache a very leaky abstraction. This is merely an annoyance when working on an individual container build but would be a complete dealbreaker at linux-distro-scale, which is what Nix operates at.