don't attempt to attach 'restarting' containers

this is an optimisation... there is no point attempting to attach a
'restarting' container - it will fail.

When the container *actually* restarts we'll get a 'Start' event, and
attach at that point.

Also, there is no point checking for 'Paused' since 'Paused' ->
'Running'.

proxy/proxy.go

@@ -132,7 +132,7 @@ func (proxy *Proxy) AttachExistingContainers() {
 			}
 			continue
 		}
-		if containerShouldAttach(container) && (container.State.Running || container.State.Paused) {
+		if containerShouldAttach(container) && container.State.Running && !container.State.Restarting {
 			proxy.attach(container, false)
 		}
 	}