Hey there.
In that case you can just issue a docker system prune command, on the OpenBalena host, as /var/lib/docker contains the OpenBalena component images (so removing the dangling images after an update should release some space).
At the time, the registry service images are being stored on the bundled S3 service data volume. Removing previous release images is currently not supported, (as it would currently break the capability of pinning a device to a past release). However, you can interact and issue delete requests to the OpenBalena registry for removing images, and reclaiming space More info. Hope that helps!