Hi Greg H: Glad to know that you were able to figure this out and thanks for sharing the rootcause. By the way, Microsoft Q&A community has a policy that "The question author cannot accept their own answer, They can only accept answers by others.". Hence request you to accept this as answer so we can close this thread. I am summarizing the issue and root cause you have found for it.
OP faced an issue where the images pulled from the Azure container service were not latest, upon investigation he had found that the fault was with docker build not with the Azure container service.