fix: pass mqtt credentials through compose

tests/test_runtime_assets.py

@@ -213,6 +213,18 @@ class ComposeRuntimeTests(unittest.TestCase):
                     published_targets.add(match.group(1))
         self.assertIn("7531", published_targets)
 
+    def test_compose_passes_all_supported_mqtt_environment_overrides(self):
+        environment = _environment_map(self.service.get("environment"))
+        for name in (
+            "MQTT_HOST",
+            "MQTT_PORT",
+            "MQTT_USERNAME",
+            "MQTT_PASSWORD",
+            "MQTT_CLIENT_ID",
+        ):
+            with self.subTest(name=name):
+                self.assertIn(name, environment)
+
     def test_compose_uses_dedicated_data_media_and_cookie_mounts(self):
         mounts = _volume_mounts(self.service.get("volumes"))
         by_target = {target: (source, read_only) for source, target, read_only in mounts}