PRs that update the Docker image should run tests on that image

[past]

Kyle tells me that we used to have this in Travis CI, but we might have left it behind after the migration to GitHub actions.

Right now PRs that modify any of the docker configuration files (e.g. #3180) fail in CI because the GitHub actions use the latest docker image from merges to the main branch. A sketch of the desired solution here would be to change .github/actions/make-in-docker/action.yml to:

image: docker://webplatformtests/wpt.fyi:${{ vars.GITHUB_REF_NAME }}

And perhaps also set tag_names: true in docker-update.yml