Change MYSQL_* to MARIADB_*

[iuriguilherme]

latest mariadb uses environment variables like MARIADB_USER MARIADB_PASSWORD MARIADB_DATABASE instead of the MYSQL prefix.

Docker compose doesn't override multiple env files when using the form MYSQL_USER: ${MARIADB_USER}

So I have to update the same value twice when sharing the env_file with the mariadb container:

MARIADB_USER: user
MYSQL_USER: user