Create multiple tags for docker image

[cwpearson]

We could change the rai_build.yml format to accept something like:

rai:
  version: 0.2
commands:
  build_image:
    image_name: 
      - c3sr/celery:4.0.2
      - c3sr/celery:latest
    dockerfile: "./Dockerfile"
    push:
      push: true
      credentials:
        username: dakkak
        password: ==AES32==PASS

multiple names under the commands.build_image.image_name field, instead of just a single one