title: Custom VM images for Google Cloud Dataflow workers
body: <p>Having skimmed the Google Cloud Dataflow documentation, my impression is that worker VMs run a specific predefined Python 2.7 environment without any option to change that. Is it possible to provide a custom VM image for the workers (built with libraries, external commands that the particular application needs). Is it possible to run Python 3 on Gcloud Dataflow?</p>