This … didn’t work as expected.
(The instructions are a bit off but I followed the update at https://meta.discourse.org/t/faster-rebuilds/40341/4.)
Should the new 2-container installation present an empty/fresh site? I was assuming it would copy all of my settings & data from my app container, but it was brand new. 
Edit: I did a restore from a backup made just before the process, and it seemed to restore everything. So probably this just needs to be made clear. 