docs: clarify Docker action pre-if metadata

[sgibler-droid]

Summary

Clarifies the Docker action metadata docs around pre-entrypoint and pre-if.

Changes

• Adds a Docker-specific runs.pre-if section under Docker container action metadata.
• Updates the runs.pre-entrypoint paragraph to link to the Docker-specific pre-if anchor instead of the JavaScript-action pre-if section.
• Adds a small Docker action example showing pre-entrypoint guarded by pre-if.

Fixes #35264.

[github-actions]

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/metadata-syntax.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16

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