Docker Nextcloud OCC command fails

[topo01]

I'm running two containers for Nextcloud and MariaDB since more than a year now.
This setup works fine.

My problem:
It seems impossible to run an OCC command. I always get the following error message:

An unhandled exception has been thrown:
Doctrine\DBAL\Exception: Failed to connect to the database: An exception occurred in the driver: SQLSTATE[HY000] [2002] php_network_getaddresses: getaddrinfo failed: Temporary failure in name resolution in /var/www/html/lib/private/DB/Connection.php:87

I have even tried setting up completely new containers just for testing, both, with docker-compose and with seperate docker containers. The result is always the same.
May this be related to the TMs Docker implementations network issue? What can I do?

Without OCC working I currently don't see a way how I can import user files in a new Nextcloud setup. I will need this in order to upgrade my TOS system from 2GB to 8GB.
So even though I have good backups I'm stuck.

Any ideas anyone?