def image_dir(image)
      relative = File.join "docker", env, image
      File.expand_path relative, Dir.pwd
    end
def image_dir(image)
      relative = File.join "docker", env, image
      File.expand_path relative, Dir.pwd
    end
Returns the path to the given image's data directory (which includes at
 minimum the Dockerfile, plus any other relevant files the user may have
 placed there).