Create a docker image + circleci builder

[pjenvey]

Setup the necessary Docker file and a rust building environment on circleci.

Not sure it's totally necessary but https://github.com/emk/rust-musl-builder is available for building very portable static rust binaries, which might be helpful down the line for deploying into generic linux Docker images.

[mostlygeek]

Dockerflow? :)

[pjenvey]

Dockerflow? :)

Yup and good timing, @mostlygeek, I was about to bug you to create https://hub.docker.com/u/mozilla/megaphone/ and get its credentials into https://circleci.com/gh/mozilla-services/megaphone

[mostlygeek]

@oremj could you set up the dockerhub / circleci stuff?

[pjenvey]

I've got a WIP branch that's already building an image:

wip2...dockerci

[oremj]

The dockerhub environment variables are all set up. Check out the dockerflow circle.yml for an example of how to use the variables.