Return unfinished items to the queue when they can be retried.

workers/dockerfilebuild.py

@@ -212,6 +212,10 @@ def process_work_items(pool):
       def complete_callback(completed):
         if completed:
           dockerfile_build_queue.complete(local_item)
+        else:
+          # We have a retryable error, add the job back to the queue
+          dockerfile_build_queue.incomplete(local_item)
+
       return complete_callback
 
     logger.debug('Sending work item to thread pool: %s' % pool)