Add corepack to base image

What does this MR do and why?

Add corepack to base image.

Also:

• Remove OS-installed yarn from HTML linting image.
• Add comments about Yarn v1 to Markdown linting image.

CC @aqualls @sarahgerman just FYI

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.