Docker: Simplify docker module by making usernames adjustable

modules/programs/docker.nix

@@ -1,8 +1,14 @@
 {
-  flake.modules.nixos.docker = {
+  config.flake.factory.docker = users: { lib, ... }: {
     virtualisation.docker = {
       enable = true;
       autoPrune.enable = true;
     };
+    users.users =
+      lib.genAttrs
+        users
+        (_: {
+          extraGroups = [ "docker" ];
+        });
   };
 }