docker: debian-bootstrap.pre allow customising of variant/url

We default to the buildd variant as most of our images are for
building. However lets give the user the ability to specify "minbase"
if they want to create a simple base image for experimentation.

Allowing the tweaking of DEB_URL means we can also bootstrap other
Debian based OS's. For example:

  make docker-binfmt-image-debian-ubuntu-bionic-arm64 \
       DEB_ARCH=arm64 DEB_TYPE=bionic \
       DEB_VARIANT=minbase DEB_URL=http://ports.ubuntu.com/

 \
       EXECUTABLE=./aarch64-linux-user/qemu-aarch64

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>