In newer versions of NixOS it's possible to use Docker directly in your /etc/nixos/configuration.nix!

Example from that page:

 { config, pkgs, ... }:
 {
   config.docker-containers = {
     hackagecompare = {
       image = "chrissound/hackagecomparestats-webserver:latest";
       ports = ["127.0.0.1:3010:3010"];
       volumes = [
         "/root/hackagecompare/packageStatistics.json:/root/hackagecompare/packageStatistics.json"
       ];
       cmd = [
         "--base-url"
         "\"/hackagecompare\""
       ];
     };
   };
 }

I've moved to this format as it's a bit cleaner and simpler to use for syncing container images than rkt wound up being.