feat(docker): add new environment variables for Docker config

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/config/helpers.ex

@@ -0,0 +1,12 @@
+defmodule Mobilizon.Service.Config.Helpers do
+  @moduledoc """
+  Provide some helpers to configuration files
+  """
+
+  @spec host_from_uri(String.t() | nil) :: String.t() | nil
+  def host_from_uri(nil), do: nil
+
+  def host_from_uri(uri) when is_binary(uri) do
+    URI.parse(uri).host
+  end
+end