Add docker instructions

[friism]

this is pending a Heroku launch slated for Aug 18th, please hold off on merging if you can

[itamarhaber]

Thanks's for the heads up @friism - will try keeping this at bay until the 18th ;)

[friism]

@itamarhaber we're ready on the Heroku end for releasing this. Docs are here: https://devcenter.heroku.com/articles/docker

I sent a couple other PRs related to this, they can be safely merged too.