c8d/authorizer: Default to docker.io
When the `ServerAddress` in the `AuthConfig` provided by the client is
empty, default to the default registry (registry-1.docker.io).
This makes the behaviour the same as with the containerd image store
integration disabled.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 2ad499f93e4b5117246ceffb8deb2bd6d9966fd2)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>