Certificates in pure and containerized environments

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

Dear Guix experts,

I am trying to move the execution of a Python script into a pure
environment, but it fails because of certificate issues for which I am
looking for a good fix.

The minimal example I came up with is:

   guix environment --pure \
           --ad-hoc python -- \
           python3 -c 'import urllib.request; print(urllib.request.urlopen("http://wwwbis.sidc.be/DATA/uset/Wlight/2003/11/UPH20031109112104.FTS"))'

This fails with the error message

    urllib.error.URLError: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate in certificate chain (_ssl.c:1108)>

I have no idea why Python's urllib checks a certificate chain for a http
URL, but that's not my subject now. A variant that works fine is

   guix environment --pure --preserve=^SSL_CERT \
           --ad-hoc python -- \
           python3 -c 'import urllib.request; print(urllib.request.urlopen("http://wwwbis.sidc.be/DATA/uset/Wlight/2003/11/UPH20031109112104.FTS"))'

which adds two environment variables to the temporary environment:

   SSL_CERT_DIR=/home/hinsen/.guix-profile/etc/ssl/certs
   SSL_CERT_FILE=/home/hinsen/.guix-profile/etc/ssl/certs/ca-certificates.crt

Since all the certificates come from Guix, I expect that I should also
be able to use

   guix environment --pure \
           --ad-hoc python nss-certs -- \
           python3 -c 'import urllib.request; print(urllib.request.urlopen("http://wwwbis.sidc.be/DATA/uset/Wlight/2003/11/UPH20031109112104.FTS"))'

but this doesn't work - same error as initially.

I am not happy with exposing an environment variable from my personal
account, as I want my run to be reproducible and portable to any machine
running Guix, Ideally, I'd even move on to a containerized environment.
Is there any way to achieve this?

Thanks in advance,
  Konrad