What is app-context supposed to be?

I need to specify the host of the app somewhere and I’m wondering whether app-context is the correct place or should I make my own environment variable?

Can anyone tell me why, in a freshly-created Luminus project, the ring dependency has :exclusions [ring/ring-jetty-adapter]? It would be handy for me to use lein-ring during dev, and I believe it depends on ring-jetty-adapter.