These are some example configuration files and notes around how to easily build Debian (and derivatives) disk images using the debos tool.
You'll need at least debos version 1.0.0+git20201203.e939090-4, which if you're using Debian is the version which ships in the bullseye Debian release.
If you're building Debian Bullseye targets, you'll need to have QEMU version >=5.0 on your build system due to changes to Bullseye's glibc package which enabled static PIE. The symptom when you have a too old QEMU is a segfault during the second stage of debootstrapping.