That makes sense. I think the real issue isn't Docker vs Nix, it is that some package managers are almost impossible to use to build reproducible images. I worked with debootstrap 10+ years ago trying to script reproducible builds and found it exceeding hard. Gentoo made it almost trivial (Google used it for ChromeOS so perhaps they felt similar). I will look into Nix.

It appears that with the proper package manager support, Docker would be fine?

I come from a hardware background and seem to be a lot more paranoid than most software folks. I would struggle to trust a build where so much is not pinned.