docker: Switch to using buildx

Part of getting ARM and AMD64 images published. I have those
working locally, but wanted to provide piecemeal PRs!

Makefile

@@ -71,26 +71,28 @@ clean:
 
 .PHONY: image/build-init
 image/build-init:
-	$(DOCKER) build \
+	DOCKER_BUILDKIT=1 $(DOCKER) buildx build \
 		--progress=$(DOCKER_BUILD_PROGRESS) \
 		-t $(IMAGE_INITCONTAINER_BRANCH) \
 		-f ./build/Dockerfile.initcontainer \
 		${IMAGE_BUILD_FLAGS_EXTRA} \
+		--load \
 		./build
 	$(DOCKER) tag "$(IMAGE_INITCONTAINER_BRANCH)" "$(IMAGE_INITCONTAINER_COMMIT)"
 	$(DOCKER) tag "$(IMAGE_INITCONTAINER_BRANCH)" "$(IMAGE_INITCONTAINER_TAG)"
 	$(DOCKER) tag "$(IMAGE_INITCONTAINER_BRANCH)" "$(IMAGE_INITCONTAINER_LATEST)"
 
 .PHONY: image/build
 image/build:
-	DOCKER_BUILDKIT=1 $(DOCKER) build \
+	DOCKER_BUILDKIT=1 $(DOCKER) buildx build \
 		--build-arg bpftraceversion=$(BPFTRACEVERSION) \
 		--build-arg GIT_ORG=$(GIT_ORG) \
 		--progress=$(DOCKER_BUILD_PROGRESS) \
 		--build-arg BUILDKIT_INLINE_CACHE=1 \
 		--cache-from "$(IMAGE_TRACERUNNER_BRANCH)" \
 		-t "$(IMAGE_TRACERUNNER_BRANCH)" \
 		-f build/Dockerfile.tracerunner \
+		--load \
 		${IMAGE_BUILD_FLAGS_EXTRA} \
 		.
 	$(DOCKER) tag "$(IMAGE_TRACERUNNER_BRANCH)" "$(IMAGE_TRACERUNNER_COMMIT)"