nixos/nar-serve: add domain option

nixos/modules/services/networking/nar-serve.nix

@@ -34,6 +34,17 @@ in
           - gs:// for binary caches stored in Google Cloud Storage
         '';
       };
+
+      domain = mkOption {
+        type = types.str;
+        default = "";
+        description = ''
+          When set, enables the feature of serving <nar-hash>.<domain>
+          on top of <domain>/nix/store/<nar-hash>-<pname>.
+
+          Useful to preview static websites where paths are absolute.
+        '';
+      };
     };
   };