> build an image from scratch with the packages you want

Using what? Using NixOS to configure a system is orthogonal to the system actually running the Nix binary. Nix/Nixpkgs provide well maintained package derivations and module configuration for the largest amount of software of any ecosystem. IMO it's far simpler than Yocto or Buildroot or the dozen OCI container builder ecosystems that go in and out of favor.