Add Docker setup and container CI workflow for the test runner

[dhruvang21]

Description

Run prepare, test, report, and cleanup in a container against a throwaway MySQL/MariaDB database. Save each step's log, junit.xml, env.json, and a Markdown summary to output/. See #274 (comment).

How bot report works: when Container Tests finishes for a PR, the bot downloads the results artifacts, combines each job's summary into one comment, and posts it on the PR. Later pushes update that same comment instead of adding new ones. If a run fails before uploading anything (for example, the image fails to build), the comment says so and links to the run. Cancelled runs are skipped.

Security

• It runs from the base repository, so it can comment on PRs from forks, but it never checks out or runs PR code. The artifacts are only read as text.
• It finds the PR from the run's commit and source repository, not from anything in the artifacts, so a PR can't redirect the comment to another PR.
• It only edits its own bot comment. A user comment that copies its hidden marker is ignored.
• @ mentions in the summaries are neutralized so nobody gets pinged, and the comment is trimmed to stay under GitHub's size limit.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: dhruvang21 <dhruvang21@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.