ci: attest published image SBOMs

.github/workflows/buildah.yml

@@ -158,4 +158,66 @@ jobs:
           tags: latest ${{ github.sha }}
           registry: ghcr.io/makeitworkcloud
           username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+  attest:
+    name: attest ${{ matrix.image }}
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      artifact-metadata: write
+      id-token: write
+    needs: [build, detect]
+    if: >-
+      needs.build.result == 'success' &&
+      ((github.event_name == 'push' && github.ref == 'refs/heads/main') ||
+      (github.event_name == 'workflow_dispatch' && inputs.mode == 'build & push'))
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        image: ${{ fromJson(needs.detect.outputs.images) }}
+
+    steps:
+      - name: Install skopeo
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y skopeo
+
+      - name: Resolve published image digest
+        id: image
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+          image_name="ghcr.io/makeitworkcloud/${{ matrix.image }}"
+          echo "$GITHUB_TOKEN" | skopeo login ghcr.io --username "${{ github.actor }}" --password-stdin
+          digest="$(skopeo inspect --format '{{.Digest}}' "docker://${image_name}:${GITHUB_SHA}")"
+          printf 'name=%s\n' "$image_name" >> "$GITHUB_OUTPUT"
+          printf 'digest=%s\n' "$digest" >> "$GITHUB_OUTPUT"
+
+      - name: Generate SBOM
+        uses: anchore/sbom-action@e22c389904149dbc22b58101806040fa8d37a610 # v0
+        with:
+          image: ${{ steps.image.outputs.name }}@${{ steps.image.outputs.digest }}
+          registry-username: ${{ github.actor }}
+          registry-password: ${{ secrets.GITHUB_TOKEN }}
+          output-file: ${{ matrix.image }}.spdx.json
+          format: spdx-json
+          syft-version: v1.51.1
+          upload-artifact: false
+
+      - name: Attest build provenance
+        uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6 # v4
+        with:
+          subject-name: ${{ steps.image.outputs.name }}
+          subject-digest: ${{ steps.image.outputs.digest }}
+          push-to-registry: true
+
+      - name: Attest SBOM
+        uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6 # v4
+        with:
+          subject-name: ${{ steps.image.outputs.name }}
+          subject-digest: ${{ steps.image.outputs.digest }}
+          sbom-path: ${{ matrix.image }}.spdx.json
+          push-to-registry: true

AGENTS.md

@@ -24,7 +24,7 @@ production publication.
 
 ## Build Workflow (`buildah.yml`)
 
-Single workflow, three jobs, all on `ubuntu-latest`.
+Single workflow, four jobs, all on `ubuntu-latest`.
 
 1. **checks** — runs pre-commit, including full-tree Gitleaks scanning,
    hadolint, and actionlint, for every `main` push, pull request, and manual
@@ -39,6 +39,10 @@ Single workflow, three jobs, all on `ubuntu-latest`.
    - on `push` to `main`, or `workflow_dispatch` with `mode=build & push`, push to GHCR with tags `latest` and `${{ github.sha }}`
 
 PRs and `workflow_dispatch` with `mode=build` build but do not push.
+4. **attest** — after an image is pushed, resolve its immutable GHCR digest,
+   generate an SPDX SBOM with Syft, and publish both SBOM and build-provenance
+   attestations. This job runs only for `main` pushes and explicit publishing
+   dispatches; it must not run for PR builds.
 
 ## Makefile
 

README.md

@@ -14,13 +14,20 @@ Container image monorepo. Each subdirectory containing a `Containerfile` is buil
 ## How It Works
 
 ```
-push to main ─▶ detect changed images ─▶ pre-commit + hadolint ─▶ buildah build ─▶ push to GHCR
+push to main ─▶ detect changed images ─▶ pre-commit + hadolint ─▶ buildah build ─▶ push to GHCR ─▶ attest digest
 ```
 
 `workflow_dispatch` accepts an optional `image` input to rebuild a single image; with no input it builds all images. Manual dispatch defaults to **build & push**; select **build** for a non-publishing validation run.
 
 The detect step uses the `Makefile` (`make changed-images` / `make list-images-json`) to enumerate directories that contain a `Containerfile`.
 
+## Supply-chain attestations
+
+After a publishing build, CI resolves the immutable digest behind the SHA tag,
+generates an SPDX JSON SBOM from that digest, and attaches both the SBOM and
+build provenance to the GHCR image. PR and non-publishing manual builds do not
+publish images or attestations.
+
 ## Adding an Image
 
 1. Create `<name>/Containerfile`