Thank you, thank you, thank you, Alin!
I tried all sorts of things, even creating a new user on my Mac with id 500 to match plone in the Docker container, and nothing worked. I could bash into the container and launch Plone with "bin/instance fg" but "bin/instance start" would always fail.
Then I read your suggestion and made separate Docker compose file lines for filestorage, blobstorage, and log. Voilà! Plone in Docker now works great with host-mounted volumes!!!