tests/docker: all add DOCKER_BUILDKIT to RUNC environment

It seems we also need to pass DOCKER_BUILDKIT as an argument to docker
itself to get the full benefit of caching.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230315174331.2959-3-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -39,7 +39,7 @@ docker-qemu-src: $(DOCKER_SRC_COPY)
 # General rule for building docker images.
 docker-image-%: $(DOCKER_FILES_DIR)/%.docker
 	  $(call quiet-command,			\
-		$(RUNC) build				\
+		DOCKER_BUILDKIT=1 $(RUNC) build		\
 		$(if $V,,--quiet)			\
 		$(if $(NOCACHE),--no-cache,		\
 			$(if $(DOCKER_REGISTRY),--cache-from $(DOCKER_REGISTRY)/qemu/$*)) \