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.
