Attest the web build so the shipped bytes can be verified

.github/workflows/attest.yml

@@ -0,0 +1,112 @@
+name: attest
+
+# Build provenance and SBOM attestation for the built site.
+#
+# Push-triggered on purpose: the attestation is a statement about a build that
+# actually happened, signed with an identity only GitHub's OIDC issuer can mint
+# for this repository. There is no way to produce one out of band, which is what
+# makes `gh attestation verify` mean anything.
+#
+# The subject is web/.next, the compiled application, because it is the only
+# thing in the tree that a build turns into shippable bytes. The Python half is
+# scripts and Modal entrypoints with no packaging, so there is nothing there to
+# sign.
+#
+# A directory has no digest, so it is packed into one tar first and the tar is
+# what gets signed.
+on:
+  push:
+    branches: ['**']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  provenance:
+    runs-on: ubuntu-latest
+    timeout-minutes: 25
+    permissions:
+      contents: read
+      id-token: write # mint the OIDC token Sigstore binds the signature to
+      attestations: write # write the signed bundle to this repository's attestation store
+    defaults:
+      run:
+        working-directory: web
+    steps:
+      # Every action is pinned to a commit SHA rather than a tag: a tag is a
+      # mutable pointer its owner can move to different code at any time, so a
+      # tag pin does not fix what actually runs here.
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      - uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: web/package-lock.json
+
+      # npm ci, not npm install: it installs exactly what package-lock.json
+      # pins, so the dependency set the SBOM records is the set that was linked
+      # into the build rather than whatever resolved that minute.
+      - name: install
+        run: npm ci
+
+      # After install, so the scan sees the resolved node_modules tree and not
+      # just the ranges in package.json. output-file carries the directory
+      # prefix because an action's paths resolve from the workspace root, not
+      # from the job's working-directory.
+      - name: sbom
+        uses: anchore/sbom-action@3ad7283483fc7af8ff2b4ea19663c2d5ca935e26 # v0.24.2
+        with:
+          path: web
+          format: spdx-json
+          output-file: web/winnow-web.spdx.json
+          syft-version: v1.51.1
+          upload-artifact: false
+          upload-release-assets: false
+          dependency-snapshot: false
+
+      - name: build
+        run: npm run build
+
+      # Sorted names, zeroed timestamps and zeroed ownership: without them the
+      # tar digest would change on every run for reasons that have nothing to do
+      # with the code, and a digest that moves on its own cannot be compared.
+      # .next/cache is the incremental build cache, not shipped output. Leaving
+      # it in would make the attested artifact several hundred megabytes of
+      # bytes nobody deploys, and would move the digest between runs that
+      # produced identical output.
+      - name: pack the build output
+        run: |
+          tar --sort=name --format=posix \
+              --mtime='UTC 1970-01-01' --owner=0 --group=0 --numeric-owner \
+              --exclude=./cache \
+              -cf winnow-web-build.tar -C .next .
+
+      # The digest the attestation will carry. Printed so the value a verifier
+      # computes locally can be compared against the run that produced it.
+      - name: artifact digest
+        run: sha256sum winnow-web-build.tar | tee artifact-digests.txt
+
+      - name: attest build provenance
+        uses: actions/attest-build-provenance@4d101475d8b20a2381f78447822ac1eab6504dd8 # v4.2.2
+        with:
+          subject-path: web/winnow-web-build.tar
+
+      - name: attest sbom
+        uses: actions/attest-sbom@c604332985a26aa8cf1bdc465b92731239ec6b9e # v4.1.0
+        with:
+          subject-path: web/winnow-web-build.tar
+          sbom-path: web/winnow-web.spdx.json
+
+      # The attested bytes themselves, so verification can be run against the
+      # exact artifact the attestation names rather than a local rebuild.
+      - name: upload attested artifacts
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: winnow-attested
+          path: |
+            web/winnow-web-build.tar
+            web/winnow-web.spdx.json
+            web/artifact-digests.txt
+          if-no-files-found: error