image_resource is not supported

[DennisDenuto]

piper does not use docker image name from image_resource (expects image instead)

task.yml:

image_resource:
  type: docker-image
  source:
    repository: repo/docker-image-name

Results in:

Using default tag: latest
repository name must have at least one component
2016/05/26 17:19:52 exit status 1

We've worked around this by temporarily adding an image: key with the value from image_resource.source.repository, but it would be nice if we didn't have to do this.