Merge pull request #1489 from dotcloud/1422-revert-upstart

Revert "docker.upstart: avoid spawning a `sh` process"

packaging/ubuntu/docker.upstart

@@ -5,4 +5,6 @@ stop on runlevel [!2345]
 
 respawn
 
-exec /usr/bin/docker -d
+script
+    /usr/bin/docker -d
+end script