ci: run docker jobs on dind-sm-standard-2 runners

[majinghe]

Type of Change

• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Refactor
• Other:

Related Issues

N/A

Summary of Changes

Follow-up to #266. Downsized the Docker build runner pool from dind-sm-standard-4 to dind-sm-standard-2:

• docker-build job in .github/workflows/ci.yml
• push-operator-images job in .github/workflows/docker.yml

All non-Docker jobs remain on sm-standard-2 (unchanged in this PR).

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit (fmt-check + clippy + test + console-lint + console-fmt-check)
• Added/updated necessary tests — N/A (CI runner labels only)
• Documentation updated (if needed) — N/A
• CHANGELOG.md updated under [Unreleased] (if user-visible change) — N/A (not user-visible)
• CI/CD passed (if applicable)

Impact

• Breaking change (CRD/API compatibility)
• Requires doc/config/deployment update
• Other impact: CI Docker jobs now require a self-hosted runner registered with the dind-sm-standard-2 label.

Verification

# Job/runner mapping review
grep -rn "runs-on" .github/workflows/

# Full local checks:
make pre-commit

Additional Notes

• Branched from latest main (59e75d3, which includes the merged ci: move workflow jobs to dind-sm-standard-4 and sm-standard-2 runners #266). Only two runs-on: lines change.