make container kill debug log readable

Signed-off-by: allencloud <allen.sun@daocloud.io>

daemon/kill.go

@@ -55,7 +55,7 @@ func (daemon *Daemon) ContainerKill(name string, sig uint64) error {
 // or not running, or if there is a problem returned from the
 // underlying kill command.
 func (daemon *Daemon) killWithSignal(container *container.Container, sig int) error {
-	logrus.Debugf("Sending %d to %s", sig, container.ID)
+	logrus.Debugf("Sending kill signal %d to container %s", sig, container.ID)
 	container.Lock()
 	defer container.Unlock()