Introduction
Nextcloud supports external storage. We will be adding to its external storage via this path:
Docker volume plugin --> rclone plugin --> rclone config with remote --> docker-compose.yaml volume section --> docker-compose.yaml services volume mounting section --> nextcloud webUI.
Steps
- Create config with whatever backend you want
rclone config - 
sudo mkdir -p /var/lib/docker-plugins/rclone/configandsudo mkdir -p /var/lib/docker-plugins/rclone/cache - Copy that conf file into the 
/var/lib/docker-plugins/rclone/configdirectory. - 
docker plugin install rclone/docker-volume-rclone:amd64 args="-v" --alias rclone --grant-all-permissionsandsudo docker plugin enable rclone - Add this into the volume section of the compose file for nextcloud. Tweak values as needed.
 
  ${volume_name}:
    driver: rclone
    driver_opts:
      remote: '${your_remote}:'
      allow_other: 'true'
      vfs_cache_mode: full
      poll_interval: 0
      umask: 000
- note, I have only tried the 
umask: 000option and nothing else. Perhaps something like776will work, but too lazy to try. 
- Mount the volume into the nextcloud service:
 
    volumes:
      - nextcloud:/var/www/html
      - ${your_remote}:/some_path_you_want
- Restart container: 
sudo docker compose restart - Install nextcloud external storage app.
Configuring External Storage (GUI) — Nextcloud latest Administration Manual latest documentation - Nextcloud --> Settings --> Administration --> External storage. Add parameters.
 
Beware
- File edit appearances will be slow, but they will happen at the end.
 - Let's hope this post will not be deleted - Creation of a directory called www
 - Have not been met with 
Transport endpoint not connectederror yet when rclone mount is managed via docker plug in, comment below if you have any. - You can fiddle with the other settings for performance in regards to 
vfs, but I don't think there is a need if you are just serving the external storage to be downloaded and uploaded. 
Demonstration
$ sudo docker exec -it nextcloud /bin/bash -c 'ls -hal /mounted/path'
total 4.0K
drwxrwxrwx  1 root     root    0 Jun 15 23:02 .
drwxrwxrwx 16 www-data root 4.0K Jun 15 22:59 ..
drwxrwxrwx  1 root     root    0 Jun 15 23:05 Folder
drwxrwxrwx  1 root     root    0 Jun 15 23:13 www
References
If you need to make a volume by itself.
I don't think you need to run the php occ command at all, nextcloud will be slower but it will eventually update the external storage.
No amount of work done in a container can change permissions, only the options at the creation of the volume can.