After re-reading the documentation cover-to-cover last night, I see in the Entering Configuration Information section some paragraphs that I missed earlier. A note says, "You cannot nest containers in the configuration file. All <container>
elements reside at the same "level" with the <containers> element. You configure nested containers by creating the containers in the required hierarchy in your code and then populating them from the appropriate <container> elements."
Also, <container> or <containers> are not a documented allowable child element of <container>
So the scenario I was attempted is not supported by the configuration schema or classes in unity at the moment.
It does seem reasonable that in a future release of Unity that a <container> element would support a <containers> child element that has <container> elements.