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

scripts/wait-for-mqtt.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+# wait-for-mqtt.sh: wait for MQTT broker from $MQTT_BROKER (default tcp://mosquitto:1883)
+set -e
+
+BROKER=${MQTT_BROKER:-tcp://mosquitto:1883}
+# strip tcp:// if present
+BROKER=${BROKER#tcp://}
+HOST=${BROKER%%:*}
+PORT=${BROKER#*:}
+if [ "$HOST" = "$PORT" ]; then
+  # no port provided
+  PORT=1883
+fi
+
+echo "Waiting for MQTT broker $HOST:$PORT..."
+while ! nc -z "$HOST" "$PORT"; do
+  echo "broker not ready, retrying..."
+  sleep 1
+done
+
+echo "Broker $HOST:$PORT is up, starting server"
+exec /usr/local/bin/server