Markdown linter

[dguo]

Hello, thanks for maintaining this Docker image. I was wondering if you would be open to adding a Markdown linter to it as a complement to HTMLProofer. I found a few options:

• markdownlint for Ruby
• markdownlint for Node
• remark-lint for Node

I imagine the Ruby one would be easier to add and would keep the image small since the image already has Ruby for HTMLProofer. However, the Node ones seem more actively maintained. Maybe that's because the Ruby one is older and is just more mature, but it also has more open issues.