[mook]

My understanding though is that nix tries to solve this globally (it manages your whole system, or your whole home directory, as opposed to docker, which has clearly demarcated separation between different images), and it doesn't reuse existing packaging (in particular the language, as in "apt install" etc.)

There's definitely advantages that way, but there's also drawbacks.

[tadfisher]

There's NixOS, which you are describing, and then there's `nix` the package manager and build system. The latter is installable on any Linux or macOS system as a normal binary, and can be used standalone as a build tool. My company uses this to standardize (and cache!) development environments across a diverse set of hardware; the environments do double-duty as hermetic CI builders as well.