chore(docker): 新增EMQX MQTT Broker配置

- 添加EMQX docker-compose配置
- 用于本地开发和测试环境

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/emqx/docker-compose.yml

@@ -0,0 +1,19 @@
+version: '3'
+services:
+  emqx:
+    image: emqx/emqx:5.8
+    container_name: emqx
+    restart: always
+    ports:
+      - "1883:1883"     # MQTT TCP
+      - "8083:8083"     # MQTT WebSocket
+      - "18083:18083"   # EMQX Dashboard
+    environment:
+      EMQX_NAME: emqx
+      EMQX_HOST: 127.0.0.1
+    volumes:
+      - emqx-data:/opt/emqx/data
+      - emqx-log:/opt/emqx/log
+volumes:
+  emqx-data:
+  emqx-log: