fix: harden GitHub Actions against supply chain attacks

[CybotTM]

Summary

• Pin all GitHub Actions references to immutable commit SHAs (prevents tag/branch force-push attacks)
• Add Dependabot configuration for automatic GitHub Actions version updates

Context

On 2026-03-19, aquasecurity/trivy-action was compromised via a tag force-push attack that exfiltrated secrets from CI runners. SHA-pinning prevents this class of attack entirely.

The netresearch org now enforces sha_pinning_required=true — workflows using tag/branch references will fail.

Ref: netresearch/ofelia#535

Test plan

• Verify CI passes with SHA-pinned actions
• Verify Dependabot creates PRs for action updates

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the security posture of the project's CI/CD workflows by implementing best practices to mitigate supply chain attacks. By enforcing immutable action references and automating dependency updates, it ensures the integrity and reliability of the build process against known vulnerabilities and future threats.

Highlights

• Supply Chain Security: All GitHub Actions references are now pinned to immutable commit SHAs, preventing tag/branch force-push attacks and enhancing the security of the CI/CD pipeline.
• Automated Updates: A Dependabot configuration has been added to automatically manage and update GitHub Actions versions, ensuring that the project benefits from the latest security patches and features.

Ignored Files

• Ignored by pattern: .github/workflows/** (2)
  • .github/workflows/auto-merge-deps.yml
  • .github/workflows/docker-publish.yml

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[gemini-code-assist]

Code Review

This pull request introduces a Dependabot configuration in .github/dependabot.yml to automate updates for GitHub Actions. The configuration correctly sets up weekly checks and groups all action updates into a single pull request, which will help manage the maintenance of the pinned action versions mentioned in the pull request description. The implementation is sound and I have no specific recommendations for changes.

[CybotTM]

Closing: this is a fork used for upstream PRs, not actively maintained by us.