fix: add registry login (#476)

## This PR

Fixes container signing release step [1]. This step was missing the
registry login step, hence signature push failed.


[1] -
https://github.com/open-feature/flagd/actions/runs/4357163255/jobs/7616231967

---------

Signed-off-by: Kavindu Dodanduwa <kavindudodanduwa@gmail.com>
Co-authored-by: Michael Beemer <beeme1mr@users.noreply.github.com>

.github/workflows/release-please.yaml

@@ -86,12 +86,19 @@ jobs:
     runs-on: ubuntu-latest
     if: ${{ needs.release-please.outputs.release_created }}
     steps:
+      - name: Log in to the Container registry
+        uses: docker/login-action@ec9cdf07d570632daeb912f5b2099cb9ec1d01e6
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Install Cosign
         uses: sigstore/cosign-installer@bd2d1189b064bcddc3903176a807dcdba72d7fd0
 
       - 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.digest }}
           cosign public-key --key env://COSIGN_PRIVATE_KEY --outfile ${{ env.PUBLIC_KEY_FILE }}
         env:
           COSIGN_PRIVATE_KEY: ${{secrets.COSIGN_PRIVATE_KEY}}