Detect the port at which the website is running in the bin/start script

The `bin/start` script is used to spin up the environment. There are two mentions of the website running on port 3020, and one test to see if the website has actually started that also uses that port. It is possible though for a student to change the port mapping in the docker-compose file, which would mean that the mentions and check would be incorrect. We should detect at which port the website runs and update the `bin/start` script.