Runs a docker registry for the use of this buildset.

This may be used for a single job running on a single node, or it may
be used at the root of a job graph so that multiple jobs running for a
single change can share the registry.  Two registry endpoints are
provided -- one is a local registry, the second is an upstream proxy.

**Role Variables**

.. zuul:rolevar:: buildset_registry_root
   :default: {{ ansible_user_dir }}/buildset_registry

   Path for the registry volumes.

**Return Values**

.. zuul:rolevar:: buildset_registry

   Information about the registry.

   .. zuul:rolevar:: host

      The host (IP address) of the registry.

   .. zuul:rolevar:: port

      The port on which the registry is listening.

   .. zuul:rolevar:: proxy_port

      The port on which the proxy is listening.

   .. zuul:rolevar:: username

      The username used to access the registry via HTTP basic auth.

   .. zuul:rolevar:: password

      The password used to access the registry via HTTP basic auth.

   .. zuul:rolevar:: cert

      The (self-signed) certificate used by the registry.