tests/docker/Makefile.include: only force SID to NOCACHE if old

Now we can check the age of a docker image we can be a little more
intelligent about re-building Sid images and only force NOCACHE if
it is "old".

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -75,8 +75,14 @@ docker-image-debian-win64-cross: docker-image-debian8-mxe
 # force a re-build of the base image if we ever need to build one of
 # its children.
 ifndef SKIP_DOCKER_BUILD
+ifeq ($(HAVE_USER_DOCKER),y)
+SID_AGE=$(shell $(DOCKER_SCRIPT) check --checktype=age --olderthan=180 --quiet qemu:debian-sid)
+ifeq ($(SID_AGE),)
+else
 docker-image-debian-sid: NOCACHE=1
 endif
+endif
+endif
 
 docker-image-debian-alpha-cross: docker-image-debian-sid
 docker-image-debian-hppa-cross: docker-image-debian-sid