add dockerhub post-merge integration

[RussTedrake]

i believe it now requires an account; need to request an edu account from dockerhub. Update: submitted request form on 2021/08/30

[JafarAbdi]

@RussTedrake Is this still blocked .? I worked on building docker images with GitHub actions & docker packages (we could make it push to docker-hub, too), I can spend some time trying to integrate it with this repo if you want

[RussTedrake]

That's mighty kind of you. The only blocker, i think, was getting my dockerhub edu account. I think the setup now should be straight-forward enough.