dockerfile support for gce_container

[geekflyer]

Description

It appears like currently this does not work:

gce_container:
  image_project: my-project
  image_name: my-custom-ubuntu-with-docker-and-KVM
  dockerfile: ./Dockerfile

Currently this will create a VM and a job that just hangs forever/never fully starts.
Would it be possible to add support for this to gce_container?

Best, Christian