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: NSascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1473192351-601-5-git-send-email-silbe@linux.vnet.ibm.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/debian-bootstrap.pre

@@ -15,6 +15,19 @@ exit_and_skip()
 if [ -z $FAKEROOT ]; then
     echo "Please install fakeroot to enable bootstraping" >&2
     exit_and_skip
+
+fi
+
+if [ -z "${DEB_ARCH}" ]; then
+    echo "Please set DEB_ARCH to choose an architecture (e.g. armhf)" >&2
+    exit_and_skip
+
+fi
+
+if [ -z "${DEB_TYPE}" ]; then
+    echo "Please set DEB_TYPE to a Debian archive name (e.g. testing)" >&2
+    exit_and_skip
+
 fi
 
 # We check in order for