nixos/docker-image: add example usage

nixos/modules/virtualisation/docker-image.nix

@@ -17,3 +17,41 @@
   # Socket activated ssh presents problem in Docker.
   services.openssh.startWhenNeeded = false;
 }
+
+# Example usage:
+#
+## default.nix
+# let
+#   nixos = import <nixpkgs/nixos> {
+#     configuration = ./configuration.nix;
+#     system = "x86_64-linux";
+#   };
+# in
+# nixos.config.system.build.tarball
+#
+## configuration.nix
+# { pkgs, config, lib, ... }:
+# {
+#   imports = [
+#     <nixpkgs/nixos/modules/virtualisation/docker-image.nix>
+#     <nixpkgs/nixos/modules/installer/cd-dvd/channel.nix>
+#   ];
+#
+#   documentation.doc.enable = false;
+#
+#   environment.systemPackages = with pkgs; [
+#     bashInteractive
+#     cacert
+#     nix
+#   ];
+# }
+#
+## Run
+# Build the tarball:
+# $ nix-build default.nix
+# Load into docker:
+# $ docker import result/tarball/nixos-system-*.tar.xz nixos-docker
+# Boots into systemd
+# $ docker run --privileged -it nixos-docker /init
+# Log into the container
+# $ docker exec -it <container-name> /run/current-system/sw/bin/bash