The Nova driver embeds a tiny HTTP client which talks with the Docker internal Rest API through a unix socket. It uses the HTTP API to control containers and fetch information about them.
The driver will fetch images from the OpenStack Image Service (Glance) and load them into the Docker file system. Images may be placed in Glance by exporting them from Docker using the 'docker save' command.
Older versions of this driver required running a private docker-registry, which would proxy to Glance. This is no longer required.
Hope the above given architecture will clear your doubt.