Speed up CI: Skip unnecessary build steps, optimize caching etc

[shamoon]

Proposed change

This PR significantly improves CI performance and simplifies the Docker build process, cuts Docker build time from ~25 minutes to ~4 minutes 🏎️

Highlights:

• Skips pnpm install and next build inside Docker when building in CI — reuses pre-built .next directory instead
• Adds graceful fallback for local builds to do usual next build
• More effective use of caching

Closes # (issue)

Type of change

• New service widget
• Bug fix (non-breaking change which fixes an issue)
• New feature or enhancement (non-breaking change which adds functionality)
• Documentation only
• Other (please explain)

Checklist:

• If applicable, I have added corresponding documentation changes.
• If applicable, I have reviewed the feature / enhancement and / or service widget guidelines.
• I have checked that all code style checks pass using pre-commit hooks and linting checks.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.

[github-actions]

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