➜  nix_images cat custom-iso.nix
{config, pkgs, ...}:
{
    imports = [
        <nixpkgs/nixos/modules/installer/cd-dvd/iso-image.nix>
    ];
    isoImage.makeEfiBootable = true;
}
➜  nix_images nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=custom-iso.nix