tests/docker: fix DOCKER_PARTIAL_IMAGES

Finger trouble in a previous clean-up inadvertently set
DEBIAN_PARTIAL_IMAGES instead of DOCKER_PARTIAL_IMAGES. Also fix the
typo to debian-9-mxe.

Fixes: 44d5a8bf5d
Signed-off-by: John Snow <jsnow@redhat.com>
[AJB: merged fix from Message-Id: <20190917185537.25417-1-jsnow@redhat.com>]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>

tests/docker/Makefile.include

@@ -7,7 +7,7 @@ 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 debian-sid
-DEBIAN_PARTIAL_IMAGES += debian8-mxe debian-9-mxe debian-ports debian-bootstrap
+DOCKER_PARTIAL_IMAGES += debian8-mxe debian9-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