title: how get logs for docker service tasks on "preparing" state
body: <p>I'm playing around now with docker 1.12, created a service and noticed there is a stage of <code>"preparing"</code> when I ran <code>"docker service tasks xxx"</code>. </p>
  
  <p>I can only guess that on this stage the images are being pulled or updated. </p>
  
  <p>My question is: how can I see the logs for this stage? Or more generally: how can I see the logs for docker service tasks?</p>