Registry cache...

It's yet one more incomprehensible Buildkit decision. The original Docker builder had a very simple cache system: it computed the layer hash and then checked the registry for its presence. Simple content-addressable caching.

Buildkit can NOT do this. Instead, it uses a single image as a dumping ground for the caches. If you have two builders using the same image, they'll step on each other's toes. GHA at least side-steps this.

But I tried the registry cache, and it didn't improve anything. So far, I was not able to get caching to work with multi-stage builds at all. There are open issues for that, dating back to 2020.

It has a braindead cache checking, I've fixed it locally and I'm cleaning it up for the upstream submission. But otherwise, it's always faster for me than Buildkit.