Ubuntu (Nix) container fix for BuildKite user (#1822)

Remapped GID/UIDs for `nixbld` users so they don't conflict with
BuildKite and made `/root` globally readable so that BuildKite users can
access it.

Verified by running `docker run -u 997:997 --rm
gcr.io/bazel-public/ubuntu2204-nix /bin/sh -c $'python3 -c "hello"'` and
ensuring it actually executed `python3` instead of giving a permission
error.
1 file changed