Is there some documentation on exactly how to do this? I've been curious about this in the past, but my search-fu is failing me...

There's a whole chapter on images (VM, docker, appimage) in the manual. I can recommend starting there. https://nixos.org/manual/nixpkgs/stable/#chap-images

The smallest example is:

    pkgs.dockerTools.buildLayeredImage {
      name = "hello";
      contents = [ pkgs.hello ];
    }