Add /nix/store to each layer.tar

To be totally consistent with the way Docker builds images we need to
include the /nix/store in the layer tarballs first.

pkgs/build-support/docker/store-path-to-layer.sh

@@ -9,6 +9,7 @@ layerPath="./layers/$layerNumber"
 echo "Creating layer #$layerNumber for $@"
 
 mkdir -p "$layerPath"
+tar --no-recursion -rf "$layerPath/layer.tar" /nix /nix/store
 tar -rpf "$layerPath/layer.tar" --hard-dereference --sort=name \
     --mtime="@$SOURCE_DATE_EPOCH" \
     --owner=0 --group=0 "$@"