dockerTools.buildLayeredImage: restore layer sharing

PR #58431 added /nix/store to each layer.tar. However, the timestamp was
not explicitly set while adding /nix and /nix/store to the archive. This
resulted in different SHA256 hashes of layer.tar between image builds.

This change sets time and owner when tar'ing /nix/store.

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

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