spike: build + key-based cosign against Forgejo registry

.forgejo/workflows/build.yml

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+# Forgejo Actions SPIKE stage 3a — build + push + Trivy + KEY-BASED cosign
+# sign/verify against Forgejo's own registry. Proves the migration's real
+# unknowns with no host-deploy setup. Copy to .forgejo/workflows/build.yml in a
+# throwaway Forgejo repo. Requires:
+#   - repo Variable: FORGEJO_REGISTRY (e.g. forge.currentbits.net)
+#   - repo Secrets:  COSIGN_PRIVATE_KEY, COSIGN_PASSWORD, COSIGN_PUBLIC_KEY
+name: spike-build
+on: [push, workflow_dispatch]
+permissions: {}
+env:
+  REGISTRY: ${{ vars.FORGEJO_REGISTRY }}
+jobs:
+  build:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - id: push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ env.REGISTRY }}/${{ github.repository }}/web:sha-${{ github.sha }}
+      - uses: aquasecurity/trivy-action@0.28.0
+        with:
+          image-ref: ${{ env.REGISTRY }}/${{ github.repository }}/web@${{ steps.push.outputs.digest }}
+          severity: CRITICAL,HIGH
+          exit-code: '1'
+          ignore-unfixed: true
+      - uses: sigstore/cosign-installer@v3
+      - name: cosign sign + verify (key-based)
+        env:
+          COSIGN_PASSWORD: ${{ secrets.COSIGN_PASSWORD }}
+          COSIGN_PRIVATE_KEY: ${{ secrets.COSIGN_PRIVATE_KEY }}
+          COSIGN_PUBLIC_KEY: ${{ secrets.COSIGN_PUBLIC_KEY }}
+          IMAGE: ${{ env.REGISTRY }}/${{ github.repository }}/web@${{ steps.push.outputs.digest }}
+        run: |
+          set -euo pipefail
+          cosign sign --key env://COSIGN_PRIVATE_KEY --yes "$IMAGE"
+          printf '%s\n' "$COSIGN_PUBLIC_KEY" > cosign.pub
+          cosign verify --key cosign.pub "$IMAGE"

.gitignore

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+cosign.key
+cosign.pub

Dockerfile

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+# Throwaway spike app (ADR-0064): serves 200 on /health:3000 so the platform's
+# built-app healthcheck passes. Copy into a Forgejo repo next to
+# .forgejo/workflows/build.yml. Not platform code — spike scaffolding.
+FROM nginx:alpine
+RUN printf 'server {\n  listen 3000;\n  location = /health { return 200 "ok\n"; }\n}\n' \
+    > /etc/nginx/conf.d/default.conf
+EXPOSE 3000