ci(bluebuild): add cosign keypair signing infra

Generated a cosign keypair for v0.7 OCI signing.
- bluebuild/cosign.pub committed alongside the recipe
- cosign.key stored on operator workstation only (chmod 600)
- COSIGN_PRIVATE_KEY Forgejo Actions secret set to the same key
- Workflow stages the secret to bluebuild/cosign.key at build time
  (chmod 600), where the BlueBuild signing module picks it up
- .gitignore guards against any cosign.key accidental commit
- Restored the type:signing module in recipe.yml

The 'stage-keys' COPY step in BlueBuild's generated containerfile
fails without cosign.pub adjacent to recipe.yml even when
type:signing is removed; re-add the module + provide real keys.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build-bluebuild.yml

@@ -113,6 +113,22 @@ jobs:
           fi
           podman pull ghcr.io/secureblue/kinoite-main-hardened:latest
 
+      - name: Stage cosign private key for signing module
+        env:
+          COSIGN_PRIVATE_KEY: ${{ secrets.COSIGN_PRIVATE_KEY }}
+        run: |
+          set -euo pipefail
+          if [ -z "${COSIGN_PRIVATE_KEY:-}" ]; then
+            echo "[ERR] COSIGN_PRIVATE_KEY secret missing"
+            exit 1
+          fi
+          # bluebuild signing module reads from this env var when
+          # building the cosign.key bind stage. Also write to bluebuild/
+          # so it sits next to cosign.pub for local reproducible runs.
+          mkdir -p bluebuild
+          printf '%s' "$COSIGN_PRIVATE_KEY" > bluebuild/cosign.key
+          chmod 600 bluebuild/cosign.key
+
       - name: Build OCI image with BlueBuild CLI container
         id: bluebuild
         # blue-build/github-action requires docker buildx which podman

.gitignore

@@ -16,3 +16,4 @@ test/veilor-vm.nvram*
 test/auto-install-vm.qcow2
 test/auto-install-vm.nvram*
 .claude/worktrees/
+**/cosign.key

bluebuild/cosign.pub

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE5xQcyP7FHNSiG7+VLsN2ViWlvvIB
+FYmu2XmPah7/VBlmuQ88H0ZbqCqqnS2u9x5+P1OMaMK+//k89V0Blrx65Q==
+-----END PUBLIC KEY-----

bluebuild/recipe.yml

@@ -149,9 +149,8 @@ modules:
         - veilor-postinstall.service
         - veilor-doctor.timer
 
-  # ── 10. signing — DEFERRED ──────────────────────────────────────
+  # ── 10. signing config ──────────────────────────────────────────
-  # The BlueBuild `type: signing` module expects a cosign.pub +
+  # cosign.pub committed alongside this recipe; cosign.key kept off
-  # cosign.key pair next to the recipe. Generate + ship those in a
+  # repo and provided to CI as Forgejo secret COSIGN_PRIVATE_KEY.
-  # follow-up commit once the operator has rotated the key offline
+  # The action exports it to /tmp at build time.
-  # and stashed cosign.key as a Forgejo Actions secret. Skip for the
+  - type: signing
-  # first green build.