Container nixpkgs path from flakes input?

1

I need to specify an old verrsion of nixpkgs for a nixos container (via container.<name>.nixpkgs which requires a path). I have that old version as an input in my flake.nix. How do I “convert” it to a path to pass to the container?

# flake.nix
{
inputs = {
  nixpkgs-old.url = "nixpkgs/18036c0be90f4e308ae3ebcab0e14aae0336fe42";
  nixpkgs.url = "nixpkgs/nixos-unstable";
};
outputs = {nixpkgs-old, nixpkgs}:
{ nixosConfigurations = {
        desktop = nixpkgs.lib.nixosSystem {
          system = "x86_64-linux";
          modules = [
             ./configuration.nix
            ];
        };
}

# configuration.nix
{
  containers.foo = {
    nixpkgs = <how to specify nixpkgs-old here?>;
    config = {};
};
}
2

never mind, I fired up nix repl and figured out each input has an outPath. So nixpkgs-old.outPath is all I need