You can build docker images with Nix (https://nixos.org/nixpkgs/manual/#sec-pkgs-dockerTools), but right now it wouldn't make much sense to use the NixOS modules (like nginx) for that. The modules in NixOS assume a systemd based environment and are thus not suitable for use in containers (in the end, most service modules generated systemd units).

What you can do, though, is use Nix to build a container image with an nginx config. You just can't use the abstractions present in NixOS as-is to generate the nginx config, you'd have to write the nginx config yourself (just as you would if you use a Dockerfile). You could perhaps re-use parts, or modify the NixOS modules, though.