docker: sort images list displayed by 'make docker'

we can now directly see different version sort consecutively.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -4,7 +4,7 @@
 
 DOCKER_SUFFIX := .docker
 DOCKER_FILES_DIR := $(SRC_PATH)/tests/docker/dockerfiles
-DOCKER_IMAGES := $(notdir $(basename $(wildcard $(DOCKER_FILES_DIR)/*.docker)))
+DOCKER_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
 DOCKER_CCACHE_DIR := $$HOME/.cache/qemu-docker-ccache