do not fork new thread for callback

[misli]

If we really need to use callback at all (i believe we do not), use string definition of callback ('module.submodule.func') and pass it to task.
In the task use django.utils.module_loading.import_string to get the actual callback function.

[twaugh]

Another possible approach is to use celery signals:

@signals.task_postrun(sender=dbs.tasks.build_image)
def task_postrun_handler(sender, **kwargs):
    ...

If I'm reading it right, task_postrun_handler() would be run in the server not the worker.
http://celery.readthedocs.org/en/latest/userguide/signals.html

[TomasTomecek]

I've implemented state change in DB via signals:
ef6bc60#diff-993fb812f6bae891a1e5603a70d7a96bR31

IMO, this could really be the easiest and most flexible solution. Good job scouting that out, Tim.