docker: ensure NOUSER for travis images

While adding the current user is a useful default behaviour for
creating new images it is not appropriate for Travis which already has
a default user.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170725133425.436-2-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/Makefile.include

@@ -70,6 +70,7 @@ 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
+docker-image-travis: NOUSER=1
 
 # Expand all the pre-requistes for each docker image and test combination
 $(foreach i,$(DOCKER_IMAGES), \