Fix test docker image selection

[radarhere]

Suggestions for merging into #161

When manylinux-2014-wheel-build runs, it attempts to pull from githubactions/ubuntu-22.04-jammy-amd64 - https://github.com/python-pillow/docker-images/actions/runs/3392741172/jobs/5650254050#step:5:7

Error response from daemon: pull access denied for githubactions/ubuntu-22.04-jammy-amd64, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

In the case of this repository, incorrectly using "githubactions" because docker isn't logged in for "Test Image Build", only afterwards in "Build image". The simplest solution would to login to docker earlier.

However, when this is running in a forked repository, there is likely no DOCKER_USERNAME secret, and the images are not uploaded to Docker Hub. So the test image can't be downloaded from the fork. In that situation, it would seem better to use the pythonpillow test image. Deriving the username from "dockerinfo" doesn't seem simplest when we want to detect if docker is not logged in.

So this PR detects the USERNAME from the secret directly instead of using "docker info". If that is missing, then it pulls the test image from pythonpillow.