Makefile: specify DOCKER_BUILDKIT when make images
Makefile: specify DOCKER_BUILDKIT when make images

When `make images` with buildmode docker-buildkit,
DOCKER_BUILDKIT=1 should be added to environments.

Signed-off-by: Lu Jingxiao <5d5b2f5e5481a78f88768664024c36195dc9ada6@huawei.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc