Slightly simplify the build

docker/kernel-build.Dockerfile

@@ -47,6 +47,7 @@ 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"
+RUN cd /out && tar zcf rootfs.tar.gz rootfs
 
 FROM scratch
 ARG DEVICE_TREE_TARGET
@@ -57,4 +58,4 @@ COPY --from=build \
 	/out/kernel/Image.gz \
 	/out/${DEVICE_TREE_TARGET}.dtb \
 	./
-COPY --from=build /out/rootfs/ ./rootfs
+COPY --from=build /out/rootfs.tar.gz ./rootfs.tar.gz