How to upgrade Nextcloud docker

First Shutdown docker-compose down
Then Backup using borgbackup of OMV
Then, adapt dockerfile for nextcloud Container and insert desired version in FROM line
Make sure to get every major version - See Nextcloud upgrade guideline below
Now, startup docker-compose again and force re-create: docker-compose up --force-recreate --build -d
Check logs for errors (docker-compose logs -f)
Check nextcloud settings for suggestions
For MariaDB upgrade, check MariaDB upgrade guide. Important command: docker exec CONTAINER_ID mariadb-upgrade -u root -p PASSWORD

Sources