Symlinks in "guix pack" and "guix shell"

[Konrad Hinsen <konrad.hinsen@fastmail.net>]

Dear Guix experts,

I am seeing inconsistent behavior with the creation of symbolic links
in containers and exports, and I wonder whether this is a bug or missing
documentation.

According to the documentation, there are no constraints on what I can
link to. In practice, some symbolic link requests have no effect, but
there is no error message either.

Example: I am trying to tell programs in a container to use the
certificates from package nss-certs, instead of requiring the user
to share /etc/ssl from the host machine. So I add 

   -S /etc/ssl=etc/ssl

to my command lines for "guix shell -C" and "guix pack" (for creating
Docker or Singularity images.

Sometimes this works, sometimes it doesn't. With "guix shell", I never
got it to work. With "guix pack", the observed behavior is random.
Adding a completely unrelated package to my container can make the
difference.

Can someone shed some light on this?

Cheers,
  Konrad