Yeah, a few years ago I remember looking into whether I could expose image layers from the engine as a volume to mount directly into the registry, but at least at the time it seemed complex, and when I write tools like this simplicity is a primary goal.

As a mitigation docker-pushmi-pullyu caches pushed layers between runs[1]. More often than not I'm only changing upper layers of previously-pushed images, so this helps a lot. Also, since everything happens locally the push phase is typically quite fast even with cache misses (especially on an SSD), especially compared to the pull phase which is usually going over the internet (or another network).

[1]: https://github.com/mkantor/docker-pushmi-pullyu/pull/19/file...