default.nix

{ config, pkgs, lib, ... }:

with import <home-manager/modules/lib/dag.nix> { inherit lib; };
with lib;
with builtins;

let
  appName = "Docker.app";
cfg = config.programs.docker;

package = pkgs.stdenv.mkDerivation rec {
  name = appName;
  src = pkgs.fetchurl {
    url = "https://download.docker.com/mac/stable/Docker.dmg";
    sha256 = "13qamjb9g1k093i2fn1x5dqlfq828w1zqyww6k9m0xpckd5fd2j8";
  };

  buildInputs = [ pkgs.undmg ];
  installPhase = ''
    source $stdenv/setup
    mkdir -pv $out/Applications/${appName}
    cp -r ./* $out/Applications/${appName}
  '';

  meta = {
    description = "Docker for Mac";
    homepage = https://docs.docker.com/docker-for-mac/;
    platforms = stdenv.lib.platforms.darwin;
  };
};

in {

  options = {
    programs.docker.enable = mkEnableOption "Docker";
  };

  config = {
    home.packages = [ package ];
    home.activation.docker = dagEntryAfter["installPackages"] (let
      home = config.home.homeDirectory;
      applications = "${home}/.nix-profile/Applications";
      source = "${applications}/${appName}";
      target = "${home}/Applications";
    in ''
      if [ -e ${target}/${appName} ]; then
        rm -r ${target}/${appName}
      fi
      osascript << EOF
        tell application "Finder"
        set mySource to POSIX file "${source}" as alias
        make new alias to mySource at POSIX file "${target}"
        set name of result to "${appName}"
      end tell
      EOF

      echo "\nNeed root to install Docker for Mac..."
      sudo cp ${package}/Applications/${appName}/Contents/Library/LaunchServices/com.docker.vmnetd 
      sudo cp ${package}/Applications/${appName}/Contents/Resources/com.docker.vmnetd.plist /Library/LaunchDaemons/
      sudo chmod 544 /Library/PrivilegedHelperTools/com.docker.vmnetd
      sudo chmod 644 /Library/LaunchDaemons/com.docker.vmnetd.plist
      sudo launchctl load /Library/LaunchDaemons/com.docker.vmnetd.plist
      open -a ${source} --hide
    '');
  };
}

Looks like you can get stable URLs as follows: https://download.docker.com/mac/stable/1.12.1.12133/Docker.dmg