Registering lightinject with itself again due to mapper collection builder.

It now fails becuse it doesn't support last registered as default ootb when instantiating cachehelper.
Castle fails on missing mapper collection builder when examine starts indexing.

src/Umbraco.Core/Composing/LightInject/LightInjectContainer.cs

@@ -20,6 +20,8 @@ namespace Umbraco.Core.Composing.LightInject
         protected LightInjectContainer(ServiceContainer container)
         {
             Container = container;
+
+            container.RegisterSingleton<IContainer>(_ => this);
         }
 
         /// <summary>