add 32bit builder

[Marcus Müller]

As far as I can tell, Ubuntu images on Dockerhub have no i686 variant that you can specify; and, Docker has no option to specify the arch you'd like (yay, docker).

So, the easiest way of going would probably be to replicate

https://github.com/tianon/docker-brew-ubuntu-core/blob/dist-i386/bionic/Dockerfile

But maybe there's a way to tell docker to use i386 base images that I just haven't figured out; so I'm actually doing something very unusual for me and ask on StackOverflow:

https://stackoverflow.com/questions/53237390/specifying-basing-on-i386-i686-container-in-dockerfile-for-usage-in-docker-on-am