docker: report hint when docker.py check fails

When a check fails we currently just report why we failed. This is not
totally helpful to people who want to boot-strap a new image. Report a
hint as to why it failed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Fam Zheng <famz@redhat.com>

tests/docker/Makefile.include

@@ -73,7 +73,8 @@ docker-binfmt-image-debian-%: $(DOCKER_FILES_DIR)/debian-bootstrap.docker
 			$(if $(EXECUTABLE),--include-executable=$(EXECUTABLE)), \
 			"BUILD","binfmt debian-$* (debootstrapped)"),		\
 		$(call quiet-command,						\
-			$(DOCKER_SCRIPT) check --quiet qemu:debian-$* $<,	\
+			$(DOCKER_SCRIPT) check --quiet qemu:debian-$* $< || 	\
+			{ echo "You will need to build $(EXECUTABLE)"; exit 1;},\
 			"CHECK", "debian-$* exists"))
 
 endif