Just my two cents, but combining both answers, wouldn't it be easier to do the following:
cd /usr/bin ; cp debootstrap qemu-debootstrap
and then modify the vars? That way, no need to edit the DEF_HTTPS_MIRROR back and forth for debian/ubuntu containers...
To be really thorough and avoid code duplication, it should be simple enough to create a debootcommon script with a run() function and import it inside both: