chore: add Cosign to sign Docker images

Signed-off-by: Nora Blomaard <n.blomaard@gmail.com>

Author: noraeb

.github/workflows/release-please.yml

@@ -8,6 +8,7 @@ on:
 
 permissions:
   contents: write
+  id-token: write
   packages: write
   pull-requests: write
 
@@ -49,6 +50,10 @@ jobs:
         with:
           username: ${{ secrets.DOCKER_HUB_USERNAME }}
           password: ${{ secrets.DOCKER_HUB_TOKEN }}
+      
+      - name: Install Cosign
+        if: ${{ steps.release.outputs.release_created }}
+        uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
 
       - name: Create custom Maven Settings.xml
         if: ${{ steps.release.outputs.release_created }}
@@ -62,3 +67,8 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Sign Docker image with Cosign
+        if: ${{ steps.release.outputs.release_created }}
+        run: |
+          DIGEST=$(docker buildx imagetools inspect lfenergy/compas-cim-mapping:${{ steps.release.outputs.version }} --format '{{.Manifest.Digest}}')
+          cosign sign --yes lfenergy/compas-cim-mapping@"${DIGEST}"

SECURITY.md

@@ -5,6 +5,21 @@ SPDX-License-Identifier: Apache-2.0
 -->
 # Security Policy
 
+## Verifying Docker images
+
+Docker images published by this project are signed using [Cosign](https://github.com/sigstore/cosign) keyless signing via [Sigstore](https://www.sigstore.dev/). Signatures are recorded in the public [Rekor](https://rekor.sigstore.dev/) transparency log — no private key is stored or required.
+
+To verify an image, install Cosign ([instructions](https://docs.sigstore.dev/cosign/system_config/installation/)) and run:
+
+```sh
+cosign verify \
+  --certificate-identity "https://github.com/com-pas/compas-cim-mapping/.github/workflows/release-please.yml@refs/heads/main" \
+  --certificate-oidc-issuer "https://token.actions.githubusercontent.com" \
+  lfenergy/compas-cim-mapping:<tag>
+```
+
+Replace `<tag>` with the specific release tag (e.g. `v0.12.1`) or `latest`.
+
 ## Reporting a Vulnerability
 
 Please go to [Security Advisories](https://github.com/com-pas/compas-cim-mapping/security/advisories) to privately report a security vulnerability,