If you have an existing data directory and wish to reset the root and user passwords, and to create a database on which the user can fully modify, perform the following steps.

First create a passwordreset.sql file:

CREATE USER IF NOT EXISTS root@localhost IDENTIFIED BY 'thisismyrootpassword';
SET PASSWORD FOR root@localhost = PASSWORD('thisismyrootpassword');
GRANT ALL ON *.* TO root@localhost WITH GRANT OPTION;
CREATE USER IF NOT EXISTS root@'%' IDENTIFIED BY 'thisismyrootpassword';
SET PASSWORD FOR root@'%' = PASSWORD('thisismyrootpassword');
GRANT ALL ON *.* TO root@'%' WITH GRANT OPTION;
CREATE USER IF NOT EXISTS myuser@'%' IDENTIFIED BY 'thisismyuserpassword';
SET PASSWORD FOR myuser@'%' = PASSWORD('thisismyuserpassword');
CREATE DATABASE IF NOT EXISTS databasename;
GRANT ALL ON databasename.* TO myuser@'%';

Adjust myuser, databasename and passwords as needed.

Then:

$ docker run --rm -v /my/own/datadir:/var/lib/mysql -v /my/own/passwordreset.sql:/passwordreset.sql:z mariadb:latest --init-file=/passwordreset.sql

On restarting the MariaDB container on this /my/own/datadir, the root and myuser passwords will be reset.