Add grep and Ruby back into Markdown linting image

What does this MR do and why?

While working on: gitlab!155014 (merged), I realize we do need grep and ruby after all (removed in !4702 (merged)).

See, for example:

• https://gitlab.com/gitlab-org/gitlab/-/jobs/6998678134 (no ruby).
• https://gitlab.com/gitlab-org/gitlab/-/jobs/6998706839 (no grep).

When both grep and ruby are added to the job, it passes: https://gitlab.com/gitlab-org/gitlab/-/jobs/6999044358.

This MR supports: technical-writing#979 (closed)

Test build job: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/6999433659.

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