FR: consolidation and/or better documentation for docker images

[shollyman]

While making a minor improvement to synthtool templates to improve python testing, I discovered that in addition to the testing docker image we build from https://github.com/googleapis/testing-infra-docker there's also an image built here in synthtool:

https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/.kokoro/docker/docs/Dockerfile

Both images are necessary for successfully testing our library repos, so this feels like an opportunity to improve how the images are built/maintained with some consolidation. If nothing else, some reference information that details where images definitions live and their associated gcr.io tags would be helpful for the next person, as I don't know what additional images may be relevant to document.