How to make a bootable ISO image from a bootable DVD?

You can use Brasero to copy a CD to an .iso; this is installed by default.

Select the option to copy a CD, select the CD drive as the source, and an image file as the target...

Or from inside Nautilus: go to "Computer", right-click on the CD drive, and select "Copy Disc...". (Or something like that, I'm using a local language system here.)