Fine, thank you very much for your help, my site is now cloned.
But remains a problems with mysql that don't want to start.
The first one was about user 'mysql' not existing :
[ERROR] Fatal error: Can't change to run as user 'mysql' ; Please check that the user exists!
solved by adding the line 'mysql:x:1001:1002::/home/mysql:/bin/sh' to the :etc/passwd file
as suggested in a forum
The second one is about memory for wich I can't find any solution :
[Note] InnoDB: Initializing buffer pool, size = 16.0M
InnoDB: mmap(17170432 bytes) failed; errno 12
[ERROR] InnoDB: Cannot allocate memory for the buffer pool
Could you help for this last problem ?
PS : I cloned the VM through the "clone VM" button on console.cloud.google.com for keeping the same credentials, would it be better to use google.bitnami.com/vms ? or to create a new civicrm instance via google.bitnami.com ?