The DOKKU_APP_TYPE has long since ceased to be the correct way to specify the builder for the application. It's only usage has been during the detection phase, specifically to ensure that the herokuish plugin injects the correct docker arguments during the build. As such, it is safe to migrate away to a property in a patch release.
Users that seek to set a specific builder should use 'dokku builder:set $APP selected' instead.
Refs #7863
Because the app-json extraction happens before the build-dir is set, it doesn't end up respecting the value, and thus the user might get the wrong app.json file.
This takes advantage of path.Join()'s default of ignoring empty paths.
