Added build target for the dataflow-runner container.

What does this merge request do and why?

This MR build a dataflow runner image to allow us to ru jobs though dataflow.

How to set up and validate locally

Please follow the newly added doc in this MR to verify the functionality as well as the doc itself.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.