Split testing container from testing-infra-docker

[kurtisvg]

Currently we use the currently use this Dockerfile for testing. However we really should have our own Dockerfile with our own dependencies here in python-docs-samples.

Here are the steps that need to be taken to do this:

1. Create a dockerfile based on python:3.6 here in the repo with the dependencies necessary for our tests (maybe in root or .kokoro?)
2. Create a Kokoro continuous job that checks if the Dockerfile has changed, and if so pushes a new version of the container
3. Update our tests to use this image instead of the current python-multi.

[tanjinP]

@kurtisvg I can take this on - looks pretty straightforward.

[tmatsuo]

Can we close this now?