Allow easy mounting of tmpdir volume for MariaDB Docker image

The MariaDB Docker image entrypoint automatically adjusts permissions for the datadir in case it is mounted as a volume.

In a production setup it is quite likely one would also want to mount the tmpdir as a volume. As the image uses a custom user ID to start the mariadbd process, the image should thus also ensure that the tmpdir has the same UID set on it, in order to allow for proper read/write permissions.