fix: register all containers for retirement on app stop
fix: register all containers for retirement on app stop

This ensures that unused docker images are _also_ removed. While this will mean a `ps:start` requires a rebuild in the case of a force-stop, it ensures we don't have unused images when destroying apps.

claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku