It is not only Docker that offers image registry services; companies such as Red Hat offer their own registry, where you can find the Red Hat Container Catalog, which hosts containerized versions of all of Red Hat's product offerings, along with containers to support its OpenShift offering.
Services such as Artifactory by JFrog offer a private Docker registry as part of their build services. There are also other Registry-as-a-Service offerings, such as Quay by CoreOS, who are now owned by Red Hat, and also services from Amazon Web Services and Microsoft Azure. We will take a look at these services when we move on to looking at Docker in the cloud.