Update debootstrap image to bullseye

Dockerfile

-FROM debian:stretch-slim
+FROM debian:bullseye-slim
 
 RUN apt-get update \
  && apt-get install -y debootstrap xz-utils tar \

debootstrap.sh

 #!/bin/sh
 
-: "${DIST:=stretch}"
+: "${DIST:=bullseye}"
 : "${VARIANT:=minbase}"
 : "${ARCH:=armhf}"
 : "${TARGET:=/image}"