title: docker service update vs docker stack deploy with existing stack
body: <p>I have a doubt in using docker swarm mode commands to update existing services after having deployed a set of services using <code>docker stack deploy</code>. 
  As far I understood every service is pinned to the SHA256 digest of the image at the time of creation, so if you rebuild and push an image (with same tag) and you try to run a <code>docker service update</code>, service image is not updated (even if SHA256 is different). On the contrary, if you run a <code>docker stack deploy</code> again, all the services are updated with the new images.
  I managed to update the service image also by using <code>docker service update --image repository/image:tag &lt;service&gt;</code>. Is this the normal behavior of these commands or is there something I didn't understood?</p>
  
  <p>I'm using Docker 17.03.1-ce</p>