Adds a buildset registry to the docker configuration.

Use this role on any host which should use the buildset registry.

**Role Variables**

.. zuul:rolevar:: buildset_registry

   Information about the registry, as returned by
   :zuul:role:`run-buildset-registry`.

   .. zuul:rolevar:: host

      The host (IP address) of the registry.

   .. zuul:rolevar:: port

      The port on which the registry 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.

.. zuul:rolevar:: buildset_registry_docker_user
   :default: {{ ansible_user }}

   The system user to configure to use the docker registry.  The
   docker configuration file for this user will be updated.  By
   default, the user Ansible is running as.

.. zuul:rolevar:: buildset_registry_namespaces
   :default: [ ('docker.io', 'https://...'), ('quay.io', ...), ('gcr.io', ...)]

   The namespaces that the buildset registry supports.  Each entry
   should be a tuple with the first elemnet being the registry host
   (usually the internet domain name) and the second being the URL the
   registry is hosted at.

   The buildset registry will be consulted first for images in these
   namespaces.  Any others will be fetched only from their upstream
   sources.

   Add any local or third-party registries necessary here.

   The default may change in the future as more general-purpose public
   registries become known.