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

Dockerfile

@@ -11,7 +11,9 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 \
 
 # Final image
 FROM alpine:3.18
-RUN apk add --no-cache ca-certificates
+RUN apk add --no-cache ca-certificates netcat-openbsd
 COPY --from=builder /app/server /usr/local/bin/server
+COPY scripts/wait-for-mqtt.sh /usr/local/bin/wait-for-mqtt.sh
+RUN chmod +x /usr/local/bin/wait-for-mqtt.sh
 EXPOSE 8080
-ENTRYPOINT ["/usr/local/bin/server"]
+ENTRYPOINT ["/usr/local/bin/wait-for-mqtt.sh"]

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

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