[niz4ts]

I didn't attend the talk, but Nix's dockerTools doesn't need to worry about layer ordering because there is no chance of a layer overwriting another (most layers just write to their respective /nix/store/hash-pkgname directories, which is guaranteed to be unique by Nix).

[utdemir]

You're right! All layers except the topmost layer consist of non-overlapping store paths. It's not one-path-per-layer as there's a hard limit on number of layers, so we smush the packages "deeper" in the dependency tree together within a single layer. There was some work recently where you could specify which paths should correspond to which layers but I can't recall the exact function.

You do have the ability to add a "customisation" layer on top of your image in case you want to create some directories or run arbitrary modifications which does end up as an extra layer which _can_ override the previous layers hence it's always the topmost. But you usually don't need it.

Source: I did a large-ish refactor to the `buildLayeredImage` functions a couple of years ago. My contribution was adding a `streamLayeredImage` function where you can "stream" a derivation to a Docker image on-the-fly without copying any files/layers in the disk.