feat: add cosign to sign the images (#137)

Using the output from the bake action, we sign every 
container image tag plus each specific digest using cosign.

Closes #136

Signed-off-by: Francesco Canovai <francesco.canovai@enterprisedb.com>
Signed-off-by: Jonathan Gonzalez V <jonathan.gonzalez@enterprisedb.com>
Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Co-authored-by: Francesco Canovai <francesco.canovai@enterprisedb.com>
Co-authored-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

.github/workflows/bake.yaml

@@ -12,6 +12,10 @@ on:
           - production
         default: testing
         description: "Choose the environment to bake the images for"
+      target:
+        type: string
+        default: ""
+        description: "A comma separated list of targets to build. If empty, all targets will be built."
 
 jobs:
   # Start by building images for testing. We want to run security checks before pushing those to production.
@@ -22,6 +26,8 @@ jobs:
       contents: read
       packages: write
       security-events: write
+      # Required by the cosign step
+      id-token: write
     outputs:
       metadata: ${{ steps.build.outputs.metadata }}
       images: ${{ steps.images.outputs.images }}
@@ -55,6 +61,7 @@ jobs:
           revision: ${{ github.sha }}
         with:
           push: true
+          targets: ${{ github.event.inputs.target }}
 
       # Get a list of the images that were built and pushed. We only care about a single tag for each image.
       - name: Generated images
@@ -62,6 +69,18 @@ jobs:
         run: |
           echo "images=$(echo '${{ steps.build.outputs.metadata }}' | jq -c '[ .[]."image.name" | sub(",.*";"") ]')" >>  "$GITHUB_OUTPUT"
 
+      # Even if we're testing we sign the images, so we can push them to production later if that's required
+      - name: Install cosign
+        uses: sigstore/cosign-installer@v3
+        # See https://github.blog/security/supply-chain-security/safeguard-container-signing-capability-actions/
+        # and https://github.com/actions/starter-workflows/blob/main/ci/docker-publish.yml for more details on
+        # how to use cosign.
+      - name: Sign images
+        run: |
+          echo '${{ steps.build.outputs.metadata }}' | \
+            jq '.[] | (."image.name" | sub(",.*";"" )) + "@" + ."containerimage.digest"' | \
+            xargs cosign sign --yes
+
   security:
     name: Security checks
     runs-on: ubuntu-latest
@@ -144,3 +163,15 @@ jobs:
           revision: ${{ github.sha }}
         with:
           push: true
+
+      - name: Install cosign
+        uses: sigstore/cosign-installer@v3
+        # See https://github.blog/security/supply-chain-security/safeguard-container-signing-capability-actions/
+        # and https://github.com/actions/starter-workflows/blob/main/ci/docker-publish.yml for more details on
+        # how to use cosign.
+      - name: Sign images
+        run: |
+          images=$(echo '${{ steps.build.outputs.metadata }}' |
+            jq '.[] | (."image.name" | sub(",.*";"" )) + "@" + ."containerimage.digest"'
+          )
+          cosign sign --yes ${images}

BUILD.md

@@ -122,6 +122,13 @@ docker run -d --rm -p 5000:5000 --name registry registry:2
 This command runs a lightweight, temporary instance of the `registry:2`
 container on port `5000`.
 
+## Image Signing Workflow
+
+Postgres operand images are securely signed with [cosign](https://github.com/sigstore/cosign)
+based on their digest through a GitHub workflow, using the
+[`cosign-installer` action](https://github.com/marketplace/actions/cosign-installer), which leverages
+[short-lived tokens issued through OpenID Connect](https://docs.github.com/en/actions/security-for-github-actions/security-hardening-your-deployments/about-security-hardening-with-openid-connect).
+
 ## Trademarks
 
 *[Postgres, PostgreSQL and the Slonik Logo](https://www.postgresql.org/about/policies/trademarks/)

README.md

@@ -118,6 +118,29 @@ docker buildx imagetools inspect <IMAGE> --format "{{ json .SBOM.SPDX }}"
 This command outputs the SBOM in JSON format, providing a detailed view of the
 software components and build dependencies.
 
+## Image Signatures
+
+CloudNativePG container images are securely signed using
+[cosign](https://github.com/sigstore/cosign), a tool within the
+[Sigstore](https://www.sigstore.dev/) ecosystem.
+This signing process is automated via GitHub Actions and leverages
+[short-lived tokens issued through OpenID Connect](https://docs.github.com/en/actions/security-for-github-actions/security-hardening-your-deployments/about-security-hardening-with-openid-connect).
+
+The token issuer is `https://token.actions.githubusercontent.com`, and the
+signing identity corresponds to a GitHub workflow executed under the
+`cloudnative-pg/postgres-containers` repository. This workflow uses the
+[`cosign-installer` action](https://github.com/marketplace/actions/cosign-installer)
+to facilitate the signing process.
+
+To verify the authenticity of an image using its digest, you can run the
+following `cosign` command:
+
+```sh
+cosign verify IMAGE \
+  --certificate-identity-regexp="^https://github.com/cloudnative-pg/postgres-containers/" \
+  --certificate-oidc-issuer="https://token.actions.githubusercontent.com"
+```
+
 ## Building Images
 
 For detailed instructions on building PostgreSQL container images, refer to the