Override the entrypoint in docker-compose.yml for the MariaDB Docker container by adding:
entrypoint: mysqld_safe --skip-grant-tables --user=mysql
The start up the Docker Compose stack:
$> docker-compose up -d
Then login to the Docker container:
$> sudo docker exec -ti docker-container-name bash
And login as root without password:
$> mysql -u root -p
Change the root password in mysql cli:
mysql> FLUSH PRIVILEGES;
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'your_secret';
mysql> FLUSH PRIVILEGES;
Logout of mysql and the Docker container (2x exit), remove the entrypoint line from the docker-compose.yml and reload the Docker Composer stack:
$> docker-compose up -d
You can now login to the MariaDB container and connect to the database with the new root password:
$> sudo docker exec -ti docker-container-name bash
$> mysql -u root -p