Pre-update lifecycle hook (#793)

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code
#649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code
#649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code
#649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code
#649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code #649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code #649

* Make watchtower skip update if pre-update lifecycle hook exits with a non-zero exit code #649

* Prevent starting new container if old one is not stopped because of lifecycle hook.

* Add null check for c.containerInfo.State in IsRunning

* Fixed that the container would not start

* Added test for preupdate

* EX_TEMPFAIL -> ExTempFail

* Added missing fuction ouput names

* Skip preupdate when container is restarting.

pkg/container/container.go

@@ -46,6 +46,13 @@ func (c Container) IsRunning() bool {
 	return c.containerInfo.State.Running
 }
 
+// IsRestarting returns a boolean flag indicating whether or not the current
+// container is restarting. The status is determined by the value of the
+// container's "State.Restarting" property.
+func (c Container) IsRestarting() bool {
+	return c.containerInfo.State.Restarting
+}
+
 // Name returns the Docker container name.
 func (c Container) Name() string {
 	return c.containerInfo.Name