added volume mount for app directory

examples/docker_features/docker-compose.yml

@@ -7,6 +7,7 @@ services:
       - 8080:8080
     volumes:
       - ~/path/to/niceguidata:/app/.nicegui
+      - ./app:/app # mount local app directory
     environment:
       - PUID=1000
       - PGID=1000