> so two docker builds can end up with the same hash but different output

The cache key includes the state of the filesystem so I don’t think that would ever be true.

Regardless, the purpose of the tool is to generate [layer] images to be reused, exactly to avoid the pitfalls of reproducible builds, isn’t it? In the context of the article, what makes builds reproducible is the shared cache.

Ah you're right, the hash wouldn't be the same but a Dockerfile could produce different outputs on different machines whereas nix will produce identical output on different machines.

It's not reproducible then, it's simply cached. It's a valid approach but there's tradeoffs of course.