docs: add Docker version discovery instructions to README

[rahulcrl]

Summary

• Keeps the <version> placeholder in the Docker pull command and explains how to fill it in
• Adds a gcloud artifacts docker tags list command (and a Releases page link) so users can discover available image tags

Why

We publish version-pinned tags (no latest), so a placeholder alone left readers guessing which version to use. Pointing them at the Releases page and a tag-list command lets them pick a current version instead of copying a hardcoded one that goes stale.

[cockroachlabs-cla-agent]

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