fix: use custom image having tox already installed

This MR reduces pull usage and CI load by using an image already in our registry with tox installed.

I use a rather big image: registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11

kinda related https://forge.extranet.logilab.fr/logilab/infra/dockerfiles/-/merge_requests/32 a MR to reduce the image size.