How do I build/run _one_ app with mix from the topdir of an umbrella project

We have a complex application setup as an umbrella project. The umbrella holds some"common/shared components as apps, two of the apps build and run as self-standing daemons.

Since we switched to distillery, we are building these two apps from the top of the tree, and some of our mix.exs settings have moved to the toplevel mix.exs. This is starting to break building from the app dirs, and that’s the only way I know to build/compile with mix.

So I am looking for a way to run something along the lines of mix --app=foo somecmd. Does mix know how to do this?