Perhaps "Docker, without containers" would be a closer way to convey what Nix does.
With Docker, images get built in a fresh/sandbox environment, and the resulting image is easy to distribute to other systems running Docker.
> I kind of hoped that the Nix language is more sane. I think there is a project of similar goals but the script is in Python?
A project quite related to "Nix (but not using a bespoke DSL)" is Guix, which uses Scheme.
Perhaps you're thinking of Bazel? Which uses a Python-like language, and (AFAIU) is much closer to "like make, but for the whole dependency graph".