Thanks Sven,
It makes sense (also because I stopped jumping around between blog posts and I read the doc from A to Z now).
I like the idea of a “backup-volume” image, I will try to setup something going in that direction (and because to plug or unplug a database in Oracle 12c it require some SQL as well I guess the backup-volume image can become my “tools” image taking care of unplugging the database and doing the backup or restore from a backup and plug the db back).
Thanks a lot for your reply.