Update docker image to build with alpine elixir

[Dkendal]

We can cut down on total build image size by using alpine elixir as a base image instead of the debian based image we currently use.

See https://cloud.google.com/community/tutorials/elixir-phoenix-on-kubernetes-google-container-engine for an example dockerfile.

[cbinners]

See https://github.com/battle-snake/battle_snake/pull/21

[Dkendal]

implemented