chore: update Node.js version consistency and optimize Docker build (#6285)

* chore: update Node.js version consistency and optimize Docker build

- Bump .nvmrc from v20.19.2 to v20.20.2 to match CI
- Narrow package.json engines to ^20 across root and packages/server
- Exclude @flowiseai/agentflow and @flowiseai/observe from Docker build
  to save ~40-50s per build (sdk packages not needed for app runtime)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: add build:docker script and fix Docker build filter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: update README files Node.js version to 20.0.0

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: yau-wd <yau.ong@workday.com>