Crane is a small read-only web application that provides enough of the docker registry API to support "docker pull". Crane does not serve the actual image files, but instead serves 302 redirects to some other location where files are being served. A base file location URL can be specified per-repository.
Crane loads its data from json files stored on disk. It does not have a database or use any other services. The json files can be generated with pulp by publishing a docker repository.
Crane is a flask app written in Python. It is very easy to deploy and has a small footprint, so it is a great way to provide a read-only "docker pull" API that redirects to a static file service.
Advanced users can configure a search appliance to support "docker search" and can setup repository protection using SSL certificates.