WPT Images in Docker Hub

[caseycarroll]

Feature Request Summary

Add WPT Server Docker image back to Docker Hub and update WPT Agent image to make it easier to set up private instances.

What's the motivation or use case for the change/feature?

I'm updating a very outdated and broken private instance of webpage test. The old private instance docker configuration pulls from webpagetest/server but that image no longer exists.

I see there has been ongoing work to update the docker configuration in https://github.com/WPO-Foundation/webpagetest/tree/master/docker/local. It would be nice to pull the docker image from Docker Hub rather than clone this entire repo.

Relates to #1706