Hey @darkweak, thanks for the reply!
I was unsuccessful setting it up as a container (as opposed to using it as a plugin). The main problem was TLS related as described in this post. I think the main issue was the fact that I'm running a root CA on my dev machine (created with mkcert), so acme.json really doesn't apply in that context, and I couldn't figure out how to make it work. Maybe I overlooked something obvious.
Anyway, since then, I gave plugin mode another go (no pun intended) and was SUCCESSFUL! I've now got it working on my dev machine, although lots of tweaks and refinements are still needed. It's been quite a learning experience, because I'm using several middlewares which add, remove, and modify headers; and there was a LOT of trial and error needed to figure out where in the middleware "chain" Souin should go.
At least I'm making progress now, so that's a good thing. Thanks for taking the time to respond to my post and also for creating Souin! 