feat: flagd image signing (#338)

## This PR

fixes #328

Introduce image signing for flagd. Signature is pushed to OCR repository
and public key will get added to release artefacts under the name
`publicKey.pub` (referred through variable PUBLIC_KEY_FILE in GH action)

**NOTE** - Require COSIGN_PRIVATE_KEY & COSIGN_PASSWORD secrets to be
created. And decide how to expose public key

---------

Signed-off-by: Kavindu Dodanduwa <kavindudodanduwa@gmail.com>

.github/workflows/release-please.yaml

@@ -7,6 +7,7 @@ env:
   REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}
   DEFAULT_GO_VERSION: 1.19.3
+  PUBLIC_KEY_FILE: publicKey.pub
 
 name: Run Release Please
 jobs:
@@ -77,6 +78,22 @@ jobs:
             COMMIT=${{ github.sha }}
             DATE=${{ steps.date.outputs.date }}
 
+      - name: Install Cosign
+        uses: sigstore/cosign-installer@main
+
+      - name: Sign the image
+        run: |
+          cosign sign --key env://COSIGN_PRIVATE_KEY ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ needs.release-please.outputs.release_tag_name }}
+          cosign public-key --key env://COSIGN_PRIVATE_KEY --outfile ${{ env.PUBLIC_KEY_FILE }}
+        env:
+          COSIGN_PRIVATE_KEY: ${{secrets.COSIGN_PRIVATE_KEY}}
+          COSIGN_PASSWORD: ${{secrets.COSIGN_PASSWORD}}
+
+      - name: Bundle release assets
+        uses: softprops/action-gh-release@v1
+        with:
+          files: ${{ env.PUBLIC_KEY_FILE }}
+
       - name: SBOM for latest image
         uses: anchore/sbom-action@06e109483e6aa305a2b2395eabae554e51530e1d # v0
         with: