ci: reduce Identity build overhead

[jaavid]

Summary

• skips docs-only changes
• avoids npm audit/fund network work during CI installs
• removes unconditional Docker --pull
• switches Docker validation to standard Buildx + free GitHub Actions cache
• uploads diagnostics only on failures
• keeps Node package-manager cache disabled because the repository currently has no package-lock.json

No paid Blacksmith Docker cache/sticky disk is enabled.

Follow-up debt: commit package-lock.json, then switch to npm ci and enable npm cache safely.

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