I guess we want to remove references to secret_key elsewhere as well:

 * in the various config files
 * in the gunicorn config file: https://github.com/dotcloud/docker-registry/blob/master/config/gunicorn_config.py#L10
 * in the README
https://github.com/dotcloud/docker-registry/blob/master/README.md#general-options
https://github.com/dotcloud/docker-registry/blob/master/README.md#what-about-a-production-environment