My plan was to create the img of the disk, and use mkisofs to create a bootable linux CD from it. However I'm stuck between 'how to turn installation into file you can make an iso' part.

Create a bootable USB stick from Linux I was keeping the best for last. If you are running Linux, you can use the dd command (as in the instructions for macOS above) to create the bootable drive.

Creating a bootable USB drive let's you get hands-on experience with Linux without necessarily overwriting your system.

We usually use Unetbootin to create a bootable thumb drive from a Linux ISO, but it turns out you don't even need a thumb drive if you want to boot the live CD on the same computer that has ...

Perhaps they referenced this guide to create a bootable Live USB on Windows, allowing them to test drive something like Ubuntu without making any changes to their PC.

Thanks to open-source gem UNetbootin, you can quickly and easily transform a flash drive into the ultimate rescue tool.

I could use diskutil to shrink the partition and then create a new partition, and copy the iso image (via dd) into that partition, and then boot Linux from the internal HD and do the upgrade.

YUMI Multiboot USB Creator allows you to create a Multi-Boot USB Flash Drive easily. Create Multiboot USB Flash Drive containing multiple operating systems, antivirus utilities, disc cloning ...