NixOS is capable of creating a Docker image given a Nix specification, by turning the Nix store into an archive. These images are quite minimal since they contain exactly the dependencies needed for your app.
NixOS also has its own container system, so your server configuration can simply specify a subconfiguration that will be spawned as a container. The containers reuse the Nix store from the host system, which means you get a fine-grained caching system.
I haven't used those things much so I don't really know how production-ready they are.
I sometimes use Docker (with Dockerfiles or DockerHub images) on NixOS as a way to run systems that I don't want to package with Nix. I've made a little Nix module for declaring Docker services as systemd services.
That's part of why I don't feel like Nix is a limitation. I have actually used NixOps to deploy NixOS configurations that include Docker services, so my feeling of Nix is that it kind of supervenes on everything else...
It's great to see NixOS and similar tools gaining ground. But personally I find that docker's configurations simpler to grok than NixOS's syntax. Plus it's interesting the granular level of control over system security and system access gaiened via docker (really mostly Linux secomp controls and process namespaces). It's kind of like having capabilities built into the OS on top of specification driven system configuration.