I actually got a little more complex since I may want to have "testing" settings on the test server. So I saved the file under a different name, which is in the manifest and is totally ignored by the app. To turn it on in developent, I made a symbolic link as the APP_local.conf file the app looks for. In a different environment (some test server) I can rename it to enable the testing settings.

The app is written so the stuff in APP_local.conf overrides what's in APP.conf, if such a file is found. You might search for the discussion from a few weeks (or months?) ago.