[Enhancement] Add support for yt-dlp plugins + add lifecycle script event for app boot (#465)

* Added new script type to pre-job startup tasks

* Updated Dockerfile to create the needful directories

* added tests

lib/pinchflat/lifecycle/user_scripts/command_runner.ex

@@ -12,6 +12,7 @@ defmodule Pinchflat.Lifecycle.UserScripts.CommandRunner do
   @behaviour UserScriptCommandRunner
 
   @event_types [
+    :app_init,
     :media_pre_download,
     :media_downloaded,
     :media_deleted