Enable buildkit for Makefile build target

This is set only if it is not already set.
This should give a little speedup to CI builds.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Makefile

@@ -134,9 +134,10 @@ binary: build ## build the linux binaries
 dynbinary: build ## build the linux dynbinaries
 	$(DOCKER_RUN_DOCKER) hack/make.sh dynbinary
 
+build: DOCKER_BUILDKIT ?= 1
 build: bundles
 	$(warning The docker client CLI has moved to github.com/docker/cli. For a dev-test cycle involving the CLI, run:${\n} DOCKER_CLI_PATH=/host/path/to/cli/binary make shell ${\n} then change the cli and compile into a binary at the same location.${\n})
-	docker build ${BUILD_APT_MIRROR} ${DOCKER_BUILD_ARGS} ${DOCKER_BUILD_OPTS} -t "$(DOCKER_IMAGE)" -f "$(DOCKERFILE)" .
+	DOCKER_BUILDKIT="${DOCKER_BUILDKIT}" docker build ${BUILD_APT_MIRROR} ${DOCKER_BUILD_ARGS} ${DOCKER_BUILD_OPTS} -t "$(DOCKER_IMAGE)" -f "$(DOCKERFILE)" .
 
 bundles:
 	mkdir bundles