Use docker multi-stage build

[pauldthomson]

Consider combining the .build and .deploy Dockerfiles into a single Dockerfile and using a multi-stage build: https://docs.docker.com/develop/develop-images/multistage-build/

[maccac]

@pauldthomson Sorry for the late reply. I took a look into it and I'm cool with this as long as we can keep the tests running that rely on external docker services running. Feel free to make put in a pull request to improve our build and release process.