Hello @pedante, welcome to the Nextcloud community! 
default webserver user in Docker community image is 33:33… (like all/most debian-like distros for www-data:www-data) it doesn’t matter which ID this user has in your host system or even if the user exists…
I don’t get what you trying to achieve? do you already have data there? if yes I would recommend you start with fresh local installation and customize step by step to the desired state. post you exact command lines, compose files and ls -al of the mounted directories to show issue…
also look at more advanced guide Nextcloud docker compose setup with Caddy (2024) also showing how to run the container with a custom non-root user