How do you build an auto-generating menu for forums?
Looking back at this, I realize I was asking two questions:

1) How do you configure a URL alias pattern for a forum topic in the format: [forums]/[container_name]/[forum_topic_name]?

Install and enable Pathauto. Then configure a URL alias pattern at admin/config/search/path/patterns in this format [node:taxonomy-forums:parents]/[node:taxonomy-forums:name]/[node:title].

2) How do you then add this Forum topic to a menu underneath a specific parent automatically?

Install and enable Menu Position. Add a Menu Position rule at: admin/structure/menu-position/add for Content Type: Forum topic and select the menu parent item for Forum topics.