wip: Add mariadb primary/secondary cluster

[juliusknorr]

docker-compose exec mariadb-master mysql -h maxscale -u my_user -pmy_password my_database
docker-compose exec mariadb-primary mysql -u my_user -pmy_password my_database
docker-compose exec mariadb-replica mysql -u my_user -pmy_password my_database

Useful log not exposed through docker compose logs:

docker-compose exec maxscale cat /var/log/maxscale/maxscale.log

• Needs to be hooked up to nextcloud containers
• Probably document as a standalone docker compose file (by just exposing the port directly)
• Customize health checks to wait for maxscale to be reported as ready until full setup is complete
• Check if there is a way to turn on the delay at runtime
• Replace master/slave to primary/replica where possible