docs: add labels parameter

[tyliec]

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)

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• [ ] Bugfix
• [ ] Feature
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Added undocumented labels parameter for container_image

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[github-actions[bot]]

This Pull Request has been automatically marked as stale because it has not had any activity for 180 days. It will be closed if no further activity occurs in 30 days. Collaborators can add an assignee to keep this open indefinitely. Thanks for your contributions to rules_docker!

[github-actions[bot]]

This PR was automatically closed because it went 30 days without a reply since it was labeled "Can Close?"