add docker directory, per tianon

library/centos

@@ -1,4 +1,4 @@
 # maintainer: The CentOS Project <[email protected]> (@CentOS)
 
-latest: git://github.com/CentOS/sig-cloud-instance-images@feaf968bb0ecdac8f3a5975a95b474818f78e8fd
-centos6: git://github.com/CentOS/sig-cloud-instance-images@feaf968bb0ecdac8f3a5975a95b474818f78e8fd
+latest: git://github.com/CentOS/sig-cloud-instance-images@feaf968bb0ecdac8f3a5975a95b474818f78e8fd docker
+centos6: git://github.com/CentOS/sig-cloud-instance-images@feaf968bb0ecdac8f3a5975a95b474818f78e8fd docker