(Tested in Linux Mint.)

Insert the CD/DVD in the drive.

Open a terminal.

Install dcfldd, if it isn’t already:

sudo apt-get install dcfldd

Figure out where your CD/DVD is mounted:

mount

(On my system it’s mounted as /dev/sr0)

Unmount it:

umount /dev/sr0

Create the ISO image:

dcfldd if=/dev/sr0 of=image.iso

In the example above, “image.iso” will be created in the current directory. You can change the “of” argument to whatever you want, e.g., different name, full path, etc.