Optimize Dockerfile (minimal release image)

[ysbaddaden]

While reviewing #2244 it occurred to me that maybe we didn't need the Elixir source dependencies once the application is compiled, and maybe a base Erlang runtime would be enough. Apparently it looks like we don't even need Erlang: mix can build a release binary and we could use a plain system image, much smaller than what we currently build.

See https://staknine.com/build-an-elixir-release-with-docker-to-deploy-anywhere/ for reference.

Related to #2223.