documentation: better explain the reproducibility offered by a Nix Flake build

Hello!

I have posted my thoughts about the topic of distroless containers:

https://blog.riemann.cc/digitalisation/2025/10/26/distroless-containers-nix-flakes-vs-fedora/

I have mentioned in on Mastodon and received in reply the following comment:

« Reproducible builds are implemented - Nix is being used to ensure that builds are reproducible, meaning the same source code and build instructions always produce identical container images. »

That's not exactly how nix works : it's more about "we ensure any change in the source code or build instructions will be detected": the resulting image is not necessarily reproducible (bit-by-bit), If it is, it's more thanks to the debian side

• https://linderud.dev/blog/nixos-is-not-reproducible/

• https://luj.fr/blog/is-nixos-truly-reproducible.html

I am no expert. I just say you may want to look into this topic.