flask: dev envs support & misc improvements (#263)

* Docker Desktop Development Environments config
* Use cache volumes for pip
* Upgrade from Python 3.7 -> Python 3.10
* Use port `8000` to avoid conflicts with Airplay on
  macOS for default Flask port `5000`
* Use `SIGINT` to gracefully stop Flask

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

flask/app/app.py

@@ -6,4 +6,4 @@ def hello():
 	return "Hello World!"
 
 if __name__ == '__main__':
-	app.run(host='0.0.0.0')
+	app.run(host='0.0.0.0', port=8000)