added main.py to docker-features example

examples/docker_features/app/main.py

@@ -0,0 +1,20 @@
+from nicegui import app, ui
+import signal
+
+signal.signal(signal.SIGINT, app.shutdown)
+signal.signal(signal.SIGTERM, app.shutdown)
+
+
+def store_name(input):
+    app.storage.general['name'] = input.value
+
+
+ui.input(label='Name', on_change=lambda i: store_name(i))
+
+
+def my_shutdown():
+    print("Shutdown has been initiated!")
+
+
+app.on_shutdown(my_shutdown)
+ui.run()