fix: fixing image delimeter (#463)

## This PR
 
Fixes image version delimiter to match digest usage [1]

`:tag` vs `@digest`

[1]. - https://docs.docker.com/engine/reference/run/#general-form

---------

Signed-off-by: Kavindu Dodanduwa <kavindudodanduwa@gmail.com>
Co-authored-by: Todd Baert <toddbaert@gmail.com>

.github/workflows/release-please.yaml

@@ -91,7 +91,7 @@ jobs:
 
       - name: Sign the image
         run: |
-          cosign sign --yes --key env://COSIGN_PRIVATE_KEY ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ needs.container-release.outputs.image_digest }}
+          cosign sign --yes --key env://COSIGN_PRIVATE_KEY ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ needs.container-release.outputs.image_digest }}
           cosign public-key --key env://COSIGN_PRIVATE_KEY --outfile ${{ env.PUBLIC_KEY_FILE }}
         env:
           COSIGN_PRIVATE_KEY: ${{secrets.COSIGN_PRIVATE_KEY}}