Dropping i386 from CI pipeline

Following a number of discussions, e.g. #239 (closed) and our last, April 6, dev meeting, I'm dropping i386 from the pipeline. Maybe I should say I'm "trying to drop" since I'm a bit outside of my comfort zone here, could you please check nothing is missing ?

(like removing stuff from yade-dev/docker* projects to avoid storing somewhere useless registry.gitlab.com/yade-dev/docker-yade:debian-*-i386 images ?)