Allow configurable `image_identifier`

[fonglh]

def create_container(image)
    image_identifier = "coursemology/evaluator-image-#{image}"
    Coursemology::Evaluator::DockerContainer.create(image_identifier, argv: container_arguments)
  end

image_identifier is hardcoded to coursemology/evaluator-image-
Allow it to be configured through a command line option or environment variable so custom Dockerhub repositories can be used.

[fonglh]

Soedar suggests that the entire identifier be customizable instead of just the Dockerhub organization.