Docker before 1.3.1 and docker-py before 0.5.3 fall back to HTTP when the HTTPS connection to the registry fails, which allows man-in-the-middle attackers to conduct downgrade attacks and obtain authentication and image data by leveraging a network position between the client and the registry to block HTTPS traffic.
https://groups.google.com/forum/#%21topic/docker-user/oYm0i3xShJU
http://lists.opensuse.org/opensuse-updates/2014-11/msg00048.html