The best way to find out why the page is not being served is to SSH into your droplet as root and then to run docker ps. You will see a list of running docker containers and names for each. To find out what is wrong with the backend container, enter docker logs cupldeploy_cuplbackend_1. This will print out log messages from the Flask application. If, for example, there is a problem connecting to the database, it will be reported here.