Thank you, especially to drmoss_ca. This just worked after disabling boot security. (Well, setting up the partitions for dual booting caused me angst, but was actually pretty straightforward).
If anyone is reading this in order to set up Docker + WebODM, be sure to read the install instructions for Docker carefully. One step is different for Linux Mint than for Ubuntu, and that caused me some head scratching. RTFM!
I have wanted to get WebODM running on my Mac Mini for a long time, and it was much easier than I would have thought, with these hints!