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

This reverts commit 24dd50490a027f01ea086eb90663d53348fa770e.

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