I have a similar problem too. I would like to use Duplicati (runs in Docker) to back up my data from Nextcloud (also runs in Docker). The data directory is assigned to the user www-data. Other users do not have "read rights", which is why Duplicati unfortunately cannot read and save the data directory.
However, Nextcloud must assign the data directory to the user "www-data", which is why a user "Duplicati" unfortunately doesn't help me any further.
Since I have very little knowledge of Linux, I have now failed at this point and have not yet been able to make a backup.
Maybe you can come up with a solution.
Thanks in advance.