streamLayeredImage: Change mode of /nix, /nix/store to 755

The change is insignificant when the owner is root. However, when it
is not root, this change is needed to allow using Nix (as an
unprivileged user) inside the container.

nixos/tests/docker-tools.nix

···
     ):
         docker.succeed(
             "docker load --input='${examples.bashLayeredWithUser}'",
-            "docker run -u somebody --rm ${examples.bashLayeredWithUser.imageName} ${pkgs.bash}/bin/bash -c 'test 555 == $(stat --format=%a /nix) && test 555 == $(stat --format=%a /nix/store)'",
+            "docker run -u somebody --rm ${examples.bashLayeredWithUser.imageName} ${pkgs.bash}/bin/bash -c 'test 755 == $(stat --format=%a /nix) && test 755 == $(stat --format=%a /nix/store)'",
             "docker rmi ${examples.bashLayeredWithUser.imageName}",
         )
 

pkgs/build-support/docker/stream_layered_image.py

···
         return ti
 
     def nix_root(ti):
-        ti.mode = 0o0555  # r-xr-xr-x
+        ti.mode = 0o0755  # rwxr-xr-x
         return ti
 
     def dir(path):