feat(roles): Parameterize hardcoded values in printing, nfs-mounts, and virtualisation roles

- printing role: Add configurable printerName, printerUri, and printerModel options
  to replace hardcoded Brother printer values
- nfs-mounts role: Add configurable server, remotePath, and mountPoint options
  to replace hardcoded NFS server IP (10.0.0.43)
- virtualisation role: Add configurable dockerUsers option as list type
  to replace hardcoded 'johno' docker group membership

All options have sensible defaults matching the original hardcoded values,
ensuring backward compatibility while allowing per-host customization.

Implements bead: nixos-configs-fkt

roles/virtualisation/default.nix

@@ -8,6 +8,11 @@ in
 {
   options.roles.virtualisation = {
     enable = mkEnableOption "Enable virtualisation";
+    dockerUsers = mkOption {
+      type = types.listOf types.str;
+      default = [ "johno" ];
+      description = "List of users to add to the docker group";
+    };
   };
 
   config = mkIf cfg.enable
@@ -15,6 +20,6 @@ in
       virtualisation.libvirtd.enable = true;
       programs.virt-manager.enable = true;
       virtualisation.docker.enable = true;
-      users.extraGroups.docker.members = [ "johno" ];
+      users.extraGroups.docker.members = cfg.dockerUsers;
     };
 }