Change backend server to Gunicorn

[aanchan]

WWW As a developer wanting to run the web application in production, I would like to update the server from the flask dev server to Gunicorn.

Some resources:

• https://gunicorn.org/
• https://testdriven.io/blog/dockerizing-flask-with-postgres-gunicorn-and-nginx/

AC A pull request with docker files updated to run gunicorn