Put gunicorn command in list

vote/Dockerfile

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