Simple data-in/data-out restart-until-success (or run-once/restart-never) pods need an easy way to receive input data. Such pods could be used for configuration generation functions, workflow steps, cron actions, etc.

There also needs to be an easy way to retrieve at least simple/small outputs, as discussed in #139, so I'm mostly not going to discuss that here.

Options:

1. Surface Docker attach to stdin and stdout
2. Another possibility would be to support an input field in the container spec, which would be fed into stdin. That approach would preclude reuse of a pod template for multiple inputs, though. Using a command-line argument would have that same problem. A URL parameter on the creation POST could potentially work.
3. Another approach, appropriate for larger inputs, could be image volumes (Image volumes and container volumes #831), though the input image would need to be built and pushed prior to launching the consuming pod.