I sync laptop - raspi. Default the folders were syncronized to a subfolder of ~ which is e.g. /home/gekr/ei23-docker/volumes/syncthing/lenovo/
Now I want to do an additional sync in an existing folder:
/home/gekr/ei23-docker/volumes/ei23/docs/docs
I found out, that syncthing uses the user www-data. So I changed the tree down form /home to existing/previous owner and the group www-data. The destination folder, of course, also with write access for www-data.
Generally, with Docker, this is because you haven’t mapped the path into the container correctly. You don’t describe any details on this part of the setup.