moved user declaration to file and invoking in the end

hosts/box/default.nix

@@ -12,15 +12,8 @@
     ../../modules/raspberrypi.nix
     ../../modules/common/networking.nix
     ../../modules/common/shell.nix
+    ./user.nix
   ];
-  users.users.boxuser = {
-    isNormalUser = true;
-    extraGroups = ["wheel" "docker" "networkmanager"]; # Enable ‘sudo’ for the user.
-    password = "boxuser";
-    packages = with pkgs; [
-      fastfetch
-    ];
-  };
   networking.hostName = "box";
   networking.firewall.enable = false;
   # This will be overridden by system/default.nix

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
+    ];
+  };
+}