tests/docker: add debian9-mxe to DEBIAN_PARTIAL_IMAGES

Another image that can't be used directly to build QEMU.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/Makefile.include

@@ -6,7 +6,8 @@ DOCKER_SUFFIX := .docker
 DOCKER_FILES_DIR := $(SRC_PATH)/tests/docker/dockerfiles
 DOCKER_DEPRECATED_IMAGES := debian
 # we don't run tests on intermediate images (used as base by another image)
-DOCKER_PARTIAL_IMAGES := debian debian8 debian9 debian10 debian8-mxe debian-ports debian-sid debian-bootstrap
+DOCKER_PARTIAL_IMAGES := debian debian8 debian9 debian10 debian-sid
+DEBIAN_PARTIAL_IMAGES += debian8-mxe debian-9-mxe debian-ports debian-bootstrap
 DOCKER_IMAGES := $(filter-out $(DOCKER_DEPRECATED_IMAGES),$(sort $(notdir $(basename $(wildcard $(DOCKER_FILES_DIR)/*.docker)))))
 DOCKER_TARGETS := $(patsubst %,docker-image-%,$(DOCKER_IMAGES))
 # Use a global constant ccache directory to speed up repetitive builds