How can I make "https://webmail.domain.com/" instead of "https://www.domain.com/roundcube" ?
This is not something you set up in Roundcube. You'll need to add a DNS entry for the new hostname (webmail.domain.com). You'll also need to set up a virtual host or an alias in whatever webserver (apache, IIS, etc.) that you use.
I found how ! Thanks a lot buddy. :)
But the "/etc/roundcube/apache.conf" (I had to uncomment "Alias /roundcube /var/lib/roundcube" the first time to make it work and nothing in apache's configuration files) is used for wich reason then ? If it's the apache that control all configurations...