None of the previous answers mention
bootcd, that seems to correspond exactly to your question. Nevertheless, I never tried it.
This is the package description:
Build an image of your running Debian System with the command bootcdwrite.
You can also build a bootcd ISO image via NFS on a remote System.
When you run your system from CD you do not need any disks. All
changes will be done in ram. To reuse this changes at next boot time
you can save them on FLOPPY with the command bootcdflopcp. If booting
from your CD-drive is not supported, booting from FLOPPY is possible.
It is possible to install a new system from the running CD with the
command bootcd2disk. Bootcd2disk can also find a target disk, format
it and make it bootable automatically. Bootcd also supports lilo,
grub, initrd, udev, lvm, transparent-compression ISO 9660 fs and
syslinux/isolinux. The included FAQ describes how bootcd can do
online or offline backups of other Linux distributions, resulting
in restore-bootcds. The bootcdbackupwizard helps to create online