busybox permission denied

[qlibp]

Using latest version of nix-portable to build flake.nix, inside a ubuntu container, as root user, all permission is assign to /root/.nix-portable/busybox/bin/busybox, still get the following error.

Error log: getting attributes of path '/root/.nix-portable/busybox/bin/busybox': Permission denied.

flake I'm trying to build is https://github.com/nix-community/nixvim/tree/main?tab=readme-ov-file#standalone-usage