Service names can't be changed

[wpf500]

#17 solved most of the stack configuration problems, but there is one more: the service names (app, api_app, etc.) are hardcoded in the router and a few other places.

If we ever wanted to change these names within the main repository, all hosters would have to update their Docker stack configurations. A purer logical separation would be to use environment variables in the router that hosters can specify. In practice, it's far more likely that we add a new service than change the name of an existing one, and that would require a change to the stack configuration in any case, so it's much less problematic than the other problems solved in #17.

Problem: service names are hardcoded in the main repo so can't be changed in this stack configuration
Fix: move service names into environment variables