shutdown: use poweroff -f instead of halt -f

This is makes fast shutdown work on Hyper-V

While at it, also clean up whitespaces. The file was mixed
tabs and spaces. Now it uses spaces.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

alpine/packages/docker/etc/init.d/docker

@@ -174,7 +174,7 @@ stop()
         eoutdent
 
 	# now terminate with extreme prejudice
-	halt -f
+	poweroff -f
 
         return 0
 }