broker: customize mqttui with username/password

hosts/containers/broker/default.nix

@@ -1,5 +1,12 @@
 { config, pkgs, lib, zentralwerk, ... }:
 
+let
+  mymqttui = pkgs.writeScriptBin "mqttui" ''
+    export MQTTUI_USERNAME=consumer
+    export MQTTUI_PASSWORD=`cat ${(builtins.head config.services.mosquitto.listeners).users.consumer.passwordFile}`
+    exec ${pkgs.mqttui}/bin/mqttui
+  '';
+in
 {
   c3d2 = {
     deployment = {
@@ -60,8 +67,16 @@
   };
 
   environment.systemPackages = with pkgs; [
-    mqttui
+    mymqttui
   ];
 
+  users.motd = ''
+    C3D2 MQTT Broker
+    ================
+
+    Use `mqttui` to inspect the data in mosquitto.
+
+  '';
+
   system.stateVersion = "22.05";
 }