nix: Provide & use overlay for packages?

I'm really grateful that you're providing nix modules for docspell - it's much easier to configure than paperless/-ng that way (-:

As I'm currently having trouble installing docspell in a container due to confusion around the `packageOverrides` solution for getting the package definitions onto nixpkgs, I feel it would be really convenient if you could provide an overlay.nix file (or attribute from release.nix) that provides a package [overlay](https://nixos.wiki/wiki/Overlays) instead of having all users install that packageOverrides configuration in their system config.

Would that be interesting to you? As I find myself with a bit of spare time, I could try to patch that in.