Skip updating containers where no local image info can be retrieved (#612)

* Revert "Image of running container no longer needed locally (#571)"

This reverts commit 6da66fb312.

* Update client.go

* fix: skip updating when no image info can be retrieved

This will allow watchtower to continue even though the image info for a
container cannot be retrieved. If this happens one warning will be emitted
and the container will be skipped, unless NoRestart or OnlyMonitor is supplied

pkg/container/container.go

@@ -221,3 +221,8 @@ func (c Container) hostConfig() *dockercontainer.HostConfig {
 
 	return hostConfig
 }
+
+// HasImageInfo returns whether image information could be retrieved for the container
+func (c Container) HasImageInfo() bool {
+	return c.imageInfo != nil
+}