Support pulling a private image; dogfood the action in CI

.github/workflows/ci.yml

@@ -38,3 +38,17 @@ jobs:
         run: |
           docker run --rm --entrypoint sh cerberus:ci -c \
             'cerberus version && semgrep --version && gitleaks version && trivy --version'
+
+  # Dogfood: run the published image on this repo through the action itself.
+  # Also exercises the private-image pull path — ghcr packages are private
+  # unless the org allows public ones, so an unauthenticated pull would 403.
+  dogfood:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: read
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./
+        with:
+          mode: "off"

README.md

@@ -50,6 +50,17 @@ one job line covers pushes and merge requests.
 Add a nightly scheduled pipeline for dependency scanning: new CVEs land in code that never changed,
 so pushes alone will not surface them.
 
+### Pulling a private image
+
+Container packages are private by default, and some organizations disallow public ones. Then the
+runner has to authenticate:
+
+- **GitLab** — add a `DOCKER_AUTH_CONFIG` CI variable (group-level):
+  `{"auths":{"ghcr.io":{"auth":"<base64 of user:token>"}}}` where the token is a GitHub PAT with
+  `read:packages`.
+- **GitHub** — nothing to do: the action logs in with the job's `GITHUB_TOKEN`. Give the job
+  `permissions: packages: read`.
+
 Configuration lives in [`cerberus.yml`](cerberus.example.yml) in the repo root — scanners, extra args,
 custom SARIF-emitting checks, gate policy.
 

action.yml

@@ -6,11 +6,13 @@ branding:
 
 inputs:
   url:
-    description: Ingest URL from the tracker's integration settings.
-    required: true
+    description: Ingest URL from the tracker's integration settings. Not needed when mode is `off`.
+    required: false
+    default: ""
   secret:
-    description: Ingest secret (store it as a repository or organization secret).
-    required: true
+    description: Ingest secret (store it as a repository or organization secret). Not needed when mode is `off`.
+    required: false
+    default: ""
   path:
     description: Directory to scan.
     required: false
@@ -23,6 +25,13 @@ inputs:
     description: Cerberus image to run.
     required: false
     default: ghcr.io/startmatter/cerberus:latest
+  registry-token:
+    description: >
+      Token used to pull the image when it is private (the default GITHUB_TOKEN
+      works for images in the same organization; needs the `packages: read`
+      permission on the job). Leave empty for a public image.
+    required: false
+    default: ${{ github.token }}
 
 runs:
   using: composite
@@ -33,6 +42,21 @@ runs:
       shell: bash
       run: git -C "${{ github.workspace }}" fetch --deepen=2 --quiet || true
 
+    # ghcr images are private unless the org allows public packages; an
+    # anonymous pull of a private image fails with 403.
+    - name: Log in to the registry
+      if: inputs.registry-token != ''
+      shell: bash
+      env:
+        REGISTRY_TOKEN: ${{ inputs.registry-token }}
+      run: |
+        registry="${{ inputs.image }}"
+        registry="${registry%%/*}"
+        case "$registry" in
+          *.*) echo "$REGISTRY_TOKEN" | docker login "$registry" -u "${{ github.actor }}" --password-stdin ;;
+          *) echo "cerberus: image has no registry host, skipping login" ;;
+        esac
+
     - name: Run Cerberus
       shell: bash
       env:

templates/gitlab-ci.yml

@@ -9,6 +9,13 @@
 #   K_SARIF_URL     — the ingest URL from the tracker's integration settings
 #   K_SARIF_SECRET  — the secret (mask it)
 #
+# If the image is private (ghcr packages are private unless the organization
+# allows public ones), add a third group variable so the runner can pull it:
+#   DOCKER_AUTH_CONFIG (type: File is not needed — plain variable):
+#     {"auths":{"ghcr.io":{"auth":"BASE64_OF_username:github_pat_with_read:packages"}}}
+#   Build the value with:
+#     printf '%s' 'USER:TOKEN' | base64
+#
 # Pushes to the default branch report (tasks are created/closed); every other
 # branch and merge request runs a read-only check that gates the pipeline.
 #