Possible; I don't have a feel for the relative likelihoods. I think the thing nix has going for it is that you can write a nix package definition without having to actually hardcode anything in and nix itself will give you the defaults to make ex. compilers be deterministic/reproducible, and automate handling flake.lock so you don't have to actually pay attention to the pins yourself. Or put differently; you can make either one reproducible, but nix is designed to help you do that while docker really doesn't care.
It's actually how nix works by default. When you pull in a dependency, you are actually pulling in a full description of how to build it. And it pulls in full descriptions of how to build its dependencies and so on.
The only reason nix isn't dog slow is that it has really strong caching so it doesn't have to build everything from source.
Docker can resolve dependencies in a very similar manner to nix, via multi-stage builds. Each FROM makes one dependency available. However, you can only have direct access to the content from one of the dependencies resolved this way. The other ones, you have to COPY over the relevant content --from at build time.
You're totally right about the underlying container image format being much more powerful than what you can leverage from a Dockerfile. That's exactly the thing that makes nix a better Docker image builder than Docker! It leverages that power to create images that properly use layers to pull in many dependencies at the same time, and in a way that they can be freely shared in a composable way across multiple different images!
A Docker FROM is essentially the equivalent of a dependency in nix... but each RUN only has access to the stuff that comes from the FROM directly above it plus content that has been COPY-ed across (and COPY-ing destroys the ability to share data with the source of the COPY). For Docker to have a similar power to nix at building Docker images, you would need to be be able to union together an arbitrary number of FROM sources to create a composed filesystem.
Even with the Dockerfile format you can union those filesystems (COPY --link).
People use the Dockerfile format because it is accessible.
You can still use "docker build" with whatever format you want, or drive it completely via API where you have the full power of the system.
If you're using Nix, that is what you are ultimately producing, it's buried under significant amounts of boilerplate and sensible defaults. Ultimately the output of Nix (called a derivation) reads a lot like a pile of references, build instructions, and checksums.