docker: add wait-for-mqtt entrypoint; remove compose override

test/docker-compose.yml

@@ -16,8 +16,8 @@ services:
       - MQTT_TOPIC=lambdaiot
     depends_on:
       - mosquitto
-    # ensure mosquitto is reachable before launching the server
-    command: sh -c "apk add --no-cache netcat-openbsd >/dev/null 2>&1 || true; until nc -z mosquitto 1883; do echo waiting for mosquitto...; sleep 1; done; /usr/local/bin/server"
+    # server image now waits for MQTT broker itself via entrypoint
+    # no command override needed
 
   mosquitto:
     image: eclipse-mosquitto:2.0