Updated registry disk docs to build off of the proper image.
@@ -62,7 +62,7 @@ device consumable by libvirt.
Example:
```
cat << END > Dockerfile
-FROM kubevirt.io:disk
+FROM kubevirt/registry-disk-v1alpha
ADD fedora25.qcow2 /disk
END
docker build -t vmdisks/fedora25:latest .
