v0.2.0: Image options

This release makes the image option a little bit more flexible. It's now more flexible, and can even be omitted if you use its default format.

This was done in order to make building and pushing using the Docker Compose plugin easier. If you specify the service option under image, you can reuse the autogenerated Docker Compose build tag. For an example, see below.

env:
  # Shared image repository configuration
  BUILDKITE_PLUGIN_DOCKER_COMPOSE_IMAGE_REPOSITORY: example.org/user/repo

steps:
  - label: Build
    plugins:
      - docker-compose#v2.3.0:
          build: drupal
    
  - label: Extract
    plugins:
      - forumone/extract#v0.2.0:
          from: /var/www/html
          image:
            # re-use the build key here
            service: drupal