Build multiplatform docker images

[podhrmic]

@cp526 this should take care of multiplatform image builds. The non-native build takes a looong time (over 3 hours), so I split it into a separate file that runs only on merge to master.

Fixes #431

[cp526]

Thanks! I'll check whether there's an ARM github runner available somewhere, but I suspect there won't be. Does Galois have one? There might be a way to set up Cerberus's CI to "ping" another repository which then builds the ARM image (on a faster machine)?

Absent that, I wonder whether, at 3 hours, the ARM docker image job takes too long to be practical. Would we be notified of CI test failures before the ARM job is finished (to ensure feedback is quick enough)?

[podhrmic]

@cp526 on a second though, the 3 hour job is probably fine - I split when the jobs are run, and the docker images are now built only after a successful merge to master. The build CI job still runs after each commit. The docker job should never fail, as it basically runs the same steps as the build job.

A good next step would be to turn this README into a script that can be called both from the build job and from the Dockerfile, ensuring an identical build config. Plus then the user can run the same script to build cerberus/cn. I am happy to do that, but I should probably wait until you have the cerberus/cn split done?