docker: debian-bootstrap.pre: print helpful message if DEB_ARCH/DEB_TYPE unset

docker: debian-bootstrap.pre: print helpful message if DEB_ARCH/DEB_TYPE unset

The debian-bootstrap image doesn't choose a default architecture and
distribution version, instead the user has to set both DEB_ARCH and
DEB_TYPE in the environment. Print a reasonably helpful message if
either of them isn't set instead of complaining about "qemu-" being
missing or erroring out because we cannot cd to the mirror URL.

Signed-off-by: Sascha Silbe <[email protected]>
Message-Id: <1473192351[email protected]>
Signed-off-by: Fam Zheng <[email protected]>