This is awesome, I've had a need for this for quite a while, and had cooked up something similar before, but will probably swap to using this.
Checking the Dockerfile this also uses qemu to run the actual ARM binaries, I really liked how easy it was to set this up back then. All I had to do was loop mount the RPi image and then chroot into it. Everything else was handled transparently by qemu. Of course, I didn't have nice things like resize, so I was limited to making small additions.