PHP translate localhost to a socket path when connecting to a mysql/mariadb database. This break when using packweb-multi-php as the socket isn't present in the containers.
This PR create a bind mount of the host /run/mysqld (=/var/run/mysqld) where this socket is present by default for the mariadb package in debian. It also change the default configuration of mysql so that it is also where the socket is put instead of /tmp/mysql.sock (the default with Oracle MySQL).
This PR was tested with an evolixised VM running Debian 10 (It worked !).