Docker does nothing to improve reproducibility in the way that the Debian (and other) folks are using the term, which means "bit-identical builds". Nix, and likewise GNU Guix, already provide a great framework for enabling reproducible builds. In the Guix manual, we say that the builds are nearly reproducible because as Debian shows, there is still plenty of work to be done, but we do eliminate a lot of sources of nondeterminism already.

Docker is the opposite of reproducible. The idea is that you (as a user) have a known tested good image you can re-use it easily without having to worry about reproducibility (which is the responsibility of the other side - whoever is producing the images).

If you could reproduce that environment easily and instantly, you wouldn't need Docker since you'd just reproduce it every time.

Debian wasn't built with reproducibility in mind, whilst NixOS was. "Backporting" features to existing systems is monotonically harder than building a new system with that feature, since we have more constraints.

Even if, hypothetically, it were trivial for NixOS to obtain perfect reproducibility, that doesn't necessarily help with making Debian reproducible in a backwards-compatible way.

There's practically reproducible and completely reproducible.

Let's say you had a bug, and are trying to see when it was introduced.

Something like the compilation time printed in a log at startup you don't really care about, the versions of each part of your build chain and all dependencies are where your focus will be. This is easier to achieve.

For NixOS it looks like there was some work on reproducible builds, but it seems to have stalled: https://github.com/NixOS/nixpkgs/pull/2281