From nix bundle - Nix Reference Manual, I can see that there are docker image bundlers in GitHub - NixOS/bundlers.
Do we have systemd-nspawn bundlers anywhere? I think you just need to generate a folder with a full filesystem in it.
Ah, what i was looking for was the nixos-container command. Also, https://github.com/NixOS/rfcs/blob/c65c8321782b40844167beb48818471f70900d9d/rfcs/0108-nixos-containers.md is very interesting.