I don't understand what's going on in the docker-compose example.

[willyfresh]

Here are my questions about it:

• what is an actual service i'd like to run? like any app? Deluge?
• is the web server just a proxy or way to access the service I'm running?
• what if the app is supposed to run on a certain port?
• what does running two services look like?
• what does running an external service look like?