docker: print warning if EXECUTABLE is not set when building debootstrap image

Building the debian-debootstrap image will usually fail if EXECUTABLE
isn't set (when using the Makefile). Warn the user in this case so
they know why it's failing.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1473192351-601-6-git-send-email-silbe@linux.vnet.ibm.com>
Signed-off-by: Fam Zheng <famz@redhat.com>