Build tar archive on release

mix.exs

@@ -11,7 +11,7 @@ defmodule Toy.MixProject do
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps(),
-      releases: [toy: [steps: [:assemble, &export_sha1/1, &export_docker_image_tag/1]]]
+      releases: [toy: [steps: [:assemble, &export_sha1/1, &export_docker_image_tag/1, :tar]]]
     ]
   end