Post test images to pull requests

When opening a pull request, it would be helpful if a GitHub Action workflow would comment on a PR with information on how to use the test the images built for a PR.

For example:

The test images have been built for this pull request.

*** List of images ***

PHP
- docker.pkg.github.com/wordpress/wpdev-docker-images/php:8.0-fpm-51
- ........

PHPUnit
- docker.pkg.github.com/wordpress/wpdev-docker-images/phpunit:5-php-5.6-fpm-51
-  ........

etc.

Here is how to test:
*** instructions to test in a `trunk` checkout ***

This will likely be easier once the container building has been migrated from Travis to GitHub Actions.