Markdown link checker can't handle the case where Dockerfiles are added or moved

[lbussell]

The auto-generated tags tables contain absolute links to the individual Dockerfiles for each image.

If you submit a PR that moves Dockerfiles like this one, then the link checker will always fail since the links are not live yet (the PR has not been merged and the files don't exist).

Related: #5818
/cc @MichaelSimons

[lbussell]

[Triage]

Some potential solution ideas:

• Use relative links in tags tables since the tags table is no longer posted to DockerHub
• Accept merging on red
• Only run the markdown link checker on a schedule so that it only checks the live markdown files.

Of these, the first seems the most promising. It would still fail when adding new Docker repos, but that is infrequent enough that it's not a big issue.