Docker and Nix are mostly orthogonal technologies though. Nix is an excellent build tool while Docker is useful as an universal software distribution format. They're really useful together because Nix is actually good at creating compact Docker images. But of course, if you don't have to worry much about distribution, it would be much nicer to stick with Nix as you've mentioned.
I use docker for most of my clients' work but for in house stuff I just use nix.