Add gcompat to markdown linting Docker image required by newer Vale
What does this MR do and why?
I noticed when trying to use Vale 3.6.1 in a pipeline job: https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/7313292303 that I got an error.
Deeper investigation showed that Vale 3.5.0 and later require gcompat
in a MUSL environment.
CC @tle_gitlab
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