docker: add MXE (M cross environment) base image for MinGW-w64

see http://mxe.cc/

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
diff --git a/tests/docker/Makefile.include b/tests/docker/Makefile.include
index 5a82836..e993e14 100644
--- a/tests/docker/Makefile.include
+++ b/tests/docker/Makefile.include
@@ -58,6 +58,7 @@
 docker-image-debian-powerpc-cross: EXTRA_FILES:=tests/docker/dockerfiles/debian-apt-fake.sh
 
 # Enforce dependancies for composite images
+docker-image-debian8-mxe: docker-image-debian8
 docker-image-debian-amd64: docker-image-debian9
 docker-image-debian-armel-cross: docker-image-debian9
 docker-image-debian-armhf-cross: docker-image-debian9
@@ -67,6 +68,8 @@
 docker-image-debian-powerpc-cross: docker-image-debian8
 docker-image-debian-ppc64el-cross: docker-image-debian9
 docker-image-debian-s390x-cross: docker-image-debian9
+docker-image-debian-win32-cross: docker-image-debian8-mxe
+docker-image-debian-win64-cross: docker-image-debian8-mxe
 
 # Expand all the pre-requistes for each docker image and test combination
 $(foreach i,$(DOCKER_IMAGES), \