Refresh "use alternative JDBC driver" doc

[chibenwa]

CF https://github.com/apache/james-project/tree/master/server/apps/jpa-app#using-alternative-jdbc-drivers

CF https://github.com/apache/james-project/tree/master/server/apps/jpa-smtp-app#using-alternative-jdbc-drivers

Working with a customer, we discover JIB classpath management changed in recent releases.

See https://github.com/GoogleContainerTools/jib/blob/master/jib-gradle-plugin/CHANGELOG.md#310

RUN echo ":/root/libs/logback-gelf-6.1.0.jar"  >> /root/jib-classpath-file

Was needed.

I believe we need to refresh the doc of using custom jars onto the classpath to add dedicated JDBC drivers.