Draft: CTRT update for "The Docker executor"

What does this MR do?

CTRT updates for https://docs.gitlab.com/runner/executors/docker.html

Related issues

gitlab#358403

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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