moved user declaration to file and invoking in the end

hosts/box/user.nix

@@ -0,0 +1,11 @@
+{ config, pkgs, lib, ...}:
+{
+users.users.boxuser = {
+    isNormalUser = true;
+    extraGroups = ["wheel" "docker" "networkmanager"]; # Enable ‘sudo’ for the user.
+    password = "boxuser";
+    packages = with pkgs; [
+    fastfetch
+    ];
+  };
+}