Use gunicorn for voting app in prod

voting-app/Dockerfile

@@ -15,4 +15,4 @@ ADD . /app
 EXPOSE 80
 
 # Define our command to be run when launching the container
-CMD ["python", "app.py"]
+CMD gunicorn app:app -b 0.0.0.0:80 --log-file - --access-logfile - --workers 4 --keep-alive 0

voting-app/requirements.txt

@@ -1,2 +1,3 @@
 Flask
-Redis
+Redis
+gunicorn