Added CACHE_DATE arg to force recreating the docker layer during docker build of tests image

Change-Id: Icbe3c76be7618c557b461227c61463cfcc34d0ac
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/docker/mk/Makefile.include b/docker/mk/Makefile.include
index 4105295..f31d2ca 100644
--- a/docker/mk/Makefile.include
+++ b/docker/mk/Makefile.include
@@ -76,6 +76,7 @@
                    --build-arg LWUI_VERSION==$(shell cat $(MKBUILD)/.dep_lightui) \
                    --build-arg NGUI_VERSION==$(shell cat $(MKBUILD)/.dep_ngui) \
                    --build-arg TESTS_VERSION==$(shell cat $(MKBUILD)/.dep_tests) \
+                   --build-arg CACHE_DATE==$(shell date -uI) \
                    $(DOCKER_ARGS) .
 
 clean: