mariadb container logging

[morucci]

The container process logs into /var/log/mariadb that require a volume to be mounted. The container image should be fixed to ensure mariadb logs into stdout/stderr.

[mhuin]

only the error log is serialized to a file, so that the fluent bit forwarder can pick these events and forward them.

We could improve the my.cnf template to only add the serialized error log file if log forwarding is enabled.