Apple Mac: it juuuuuust works.

As others have commented, Nextcloud provide an all-in-one docker set up. I managed to follow the instructions and get it working.

However, in the end, I wanted this to replace my Dropbox subscription, and my files and reliable access to them are important to me. Given that, and my relatively low skill level, I didn’t want to futz with troubleshooting failed updates and server issues, so I just went with a Hetzner storage share, which is their managed nextcloud subscription: https://www.hetzner.com/storage/storage-share