Simple change for more efficient and less error-prone container var assignment

I understand that you are only assigning the container var to match the same action as prototype ($(selector)), but it doesn't make sense in jQuery since the container element is already assigned to `container_div`.

This just makes it so you don't rely on the ID and just assign the var directly, it still matches the Prototype version in terms of form. I ran into a bug where the code would break because it could not find the container via ID since it was not yet registered in the visible DOM.