you create a nextcloud-mysql.cnf file on your host and map it via volume into the container.
similar to: Cannot Hook Into PHP Folder via Docker - #2 by Reiner_Nippes
you can find this also in the mariadb readme on docker hub: Docker
Using a custom MySQL configuration file
The startup configuration is specified in the file /etc/mysql/my.cnf, and that file in turn includes any files found in the /etc/mysql/conf.d directory that end with .cnf. Settings in files in this directory will augment and/or override settings in /etc/mysql/my.cnf. If you want to use a customized MySQL configuration, you can create your alternative configuration file in a directory on the host machine and then mount that directory location as /etc/mysql/conf.d inside the mariadb container.
If /my/custom/config-file.cnf is the path and name of your custom configuration file, you can start your mariadb container like this (note that only the directory path of the custom config file is used in this command):
$ docker run --name some-mariadb -v /my/custom:/etc/mysql/conf.d -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mariadb:tagThis will start a new container some-mariadb where the MariaDB instance uses the combined startup settings from /etc/mysql/my.cnf and /etc/mysql/conf.d/config-file.cnf, with settings from the latter taking precedence.