Add newline to docker-image-tag release file

mix.exs

@@ -86,7 +86,7 @@ defmodule Toy.MixProject do
 
   defp export_docker_image_tag(rel) when is_struct(rel, Mix.Release) do
     with image_tag when not is_nil(image_tag) <- System.get_env("DOCKER_IMAGE_TAG"),
-         :ok <- rel |> write_in_rel_path("docker-image-tag", image_tag) do
+         :ok <- rel |> write_in_rel_path("docker-image-tag", "#{image_tag}\n") do
       rel
     else
       nil -> rel