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

tests/docker/Makefile.include

@@ -44,6 +44,9 @@ docker-image: ${DOCKER_TARGETS}
 
 # General rule for building docker images
 docker-image-%: $(DOCKER_FILES_DIR)/%.docker
+	@if test "$@" = docker-image-debian-bootstrap -a -z "$(EXECUTABLE)"; then \
+		echo WARNING: EXECUTABLE is not set, debootstrap may fail. 2>&1 ; \
+	fi
 	$(call quiet-command,\
 		$(SRC_PATH)/tests/docker/docker.py build qemu:$* $< \
 		$(if $V,,--quiet) $(if $(NOCACHE),--no-cache) \