Rewrite makefile to use out/ dir

docker/kernel-build.Dockerfile

@@ -1,6 +1,6 @@
 ARG TAG=dev
 ARG DOCKER_IMAGE_ROOT=monok8s
-FROM --platform=$BUILDPLATFORM ${DOCKER_IMAGE_ROOT}/build-base:${TAG}
+FROM --platform=$BUILDPLATFORM ${DOCKER_IMAGE_ROOT}/build-base:${TAG} AS build
 
 ARG NXP_VERSION
 ARG ARCH
@@ -46,3 +46,14 @@ RUN depmod -b /out/rootfs $(ls /out/rootfs/lib/modules/)
 
 RUN find /out/kernel -name "${DEVICE_TREE_TARGET}.dtb" -exec cp {} /out/ \;
 RUN test -f "/out/${DEVICE_TREE_TARGET}.dtb"
+
+FROM scratch
+ARG DEVICE_TREE_TARGET
+
+COPY --from=build \
+	/out/kernel/System.map \
+	/out/kernel/.config \
+	/out/kernel/Image.gz \
+	/out/${DEVICE_TREE_TARGET}.dtb \
+	./
+COPY --from=build /out/rootfs/ ./rootfs