[SOLVED - OK to remove this]
SOLUTION: I had mistakenly assigned the NAS folder /REACTOR instead of REACTOR/DATA to the '/var/reactor' volume when setting up the Docker Container for MSR. Once rectified, all Rulesets reappeared like magic.
Speaking of finding missing items, I've consulted the Docs under "Installation" and come up empty on solving a mini-crisis I got myself into tonight after rebuilding my RAID volume. On my Synology NAS > Docker > MSR setup, I had taken pains to backup the entirety of the shared REACTOR folder contents. That went well.
After successfully building my new RAID array (with larger, more reliable HDDs), I restored all the REACTOR folder contents in the same (top-level) location on my NAS. It was time to reinstall Docker, download the Reactor 'latest' image, then get the Container going. Again, all went smoothly.
Here's my problem: MSR latest-21307-1746e27 is running fine, and although it lives among all the restored folders and subfolders - full of my existing Rulesets, etc. - the UI shows none of them. Instead, I get the classic, "This rule set is empty. Click "Add Rule" to add a rule to it."
Q: How can I force a fresh MSR installation to 'see' my existing settings?
THANKS for any guidance! I'm a little scared now, and all sphincters still unclenching from the NAS HDD replacement surgery.