Add cosign image signing and SLSA provenance to container images

Summary

The browserless container images published to GHCR (ghcr.io/browserless/chromium, ghcr.io/browserless/chrome, etc.) are currently unsigned and ship without build provenance. Adding Sigstore cosign signing and SLSA provenance attestations would allow consumers to cryptographically verify image authenticity and integrity.

Motivation

Many organizations running Kubernetes enforce image verification policies via admission controllers (Kyverno, OPA Gatekeeper, Sigstore Policy Controller). Without signatures or provenance, these consumers must either:

1. Exempt browserless images from verification policies (weakening their security posture)
2. Mirror images to an internal registry and re-sign them
3. Avoid using the official images entirely

This is an increasing barrier to adoption in enterprise and security-conscious environments.

Proposed Changes

The existing docker-publish-stable.yml workflow already uses docker/build-push-action@v7 with GHCR — all the infrastructure needed is already in place. The change is minimal:

Option A: Cosign keyless signing (~10 lines)

- uses: sigstore/cosign-installer@v3

- name: Sign container images
  run: |
    cosign sign --yes \
      ghcr.io/browserless/chromium:${{ env.TAG }}@${{ steps.build-chromium.outputs.digest }}

Repeat for each of the 9 images. Keyless signing uses GitHub Actions' OIDC identity via Fulcio — no key management required.

Option B: GitHub native attestations (~5 lines per image)

- uses: actions/attest-build-provenance@v2
  with:
    subject-name: ghcr.io/browserless/chromium
    subject-digest: ${{ steps.build-chromium.outputs.digest }}
    push-to-registry: true

Option C: Both (recommended)

Cosign signatures for broad tooling compatibility + GitHub attestations for SLSA provenance. Together, ~20 lines of additional YAML across the publish workflow.

Verification

Consumers would then be able to verify images:

# Cosign
cosign verify ghcr.io/browserless/chromium:v2.24.3 \
  --certificate-identity-regexp="https://github.com/browserless/browserless/" \
  --certificate-oidc-issuer="https://token.actions.githubusercontent.com"

# GitHub attestations
gh attestation verify oci://ghcr.io/browserless/chromium:v2.24.3 \
  --owner browserless

References

• Sigstore cosign
• GitHub artifact attestations
• SLSA framework
• Kyverno image verification