This is almost certainly the correct default for Dokku. While it's a BC break and might cause an increase in memory usage, the api is mostly internal and therefore this is safe to use.
In some cases, we might hold onto intermediate images until the next deploy. While these images are generally part of newer images - and are therefore cleaned up when the child images are no longer in use - they cruft up the 'docker image ls' output, causing some folks to believe Dokku is not cleaning up images.
Refs #6272
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.
