builder: pass DOCKER_BUILDKIT to enable buildkit in tests

Signed-off-by: Tibor Vass <tibor@docker.com>

Makefile

@@ -31,6 +31,7 @@ DOCKER_ENVS := \
 	-e DOCKER_BUILD_ARGS \
 	-e DOCKER_BUILD_GOGC \
 	-e DOCKER_BUILD_PKGS \
+	-e DOCKER_BUILDKIT \
 	-e DOCKER_BASH_COMPLETION_PATH \
 	-e DOCKER_CLI_PATH \
 	-e DOCKER_DEBUG \

hack/make/.integration-test-helpers

@@ -84,6 +84,7 @@ test_env() {
 		env -i \
 			DEST="$ABS_DEST" \
 			DOCKER_API_VERSION="$DOCKER_API_VERSION" \
+			DOCKER_BUILDKIT="$DOCKER_BUILDKIT" \
 			DOCKER_INTEGRATION_DAEMON_DEST="$DOCKER_INTEGRATION_DAEMON_DEST" \
 			DOCKER_TLS_VERIFY="$DOCKER_TEST_TLS_VERIFY" \
 			DOCKER_CERT_PATH="$DOCKER_TEST_CERT_PATH" \