This feature is actively developed. It might significantly change in the future.
How to store an image in the registry
To store an image in the registry, you need to build a docker image with one /image directory in which you should put an image with an arbitrary name.
The image can be in either qcow2 or raw format.
Example of a Dockerfile:
FROM scratch
ADD https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-amd64.img /disk/jammy-server-cloudimg-amd64.img