Add $MARIADB_MYSQL_LOCALHOST_USER -> CREATE USER mysql@localhost

Monitoring and backups require authentication.

The healthcheck script as a executed within container is a perfect example. Having another password to manage on a privileged account is an additional maintaince burden. With unix_socket authentication this provides easy access for a user that will already have direct access to database files.

The security is documented in https://mariadb.com/kb/en/authentication-plugin-unix-socket/#is-it-secure