I use linuxserver.io’s docker containers for both mariadb and nextcloud on UnRaid host. Here is what I had to do. Copy the script from @YannicK_SupRavII, and create an executable .sh file in mariadb’s “config” folder which is mapped to “appdata” on unraid host.
Go into mariadb container terminal and change your directory to “config” folder, and simply execute the script you created. It will ask for your username, password, and database name.