[docker] Fix a bug causing cosign not signing images

This commits fixes a bug that causes that cosign v2
doesn't sign the images provided by docker_meta.tags.

Signed-off-by: Jose Javier Merchante <jjmerchante@bitergia.com>

.github/workflows/docker-image.yml

@@ -94,7 +94,7 @@ jobs:
 
       - name: Sign image with a key
         run: |
-          cosign sign --yes --key env://COSIGN_PRIVATE_KEY "${TAGS}@${DIGEST}"
+          echo "${TAGS}" | xargs -I {} cosign sign -y -r --key env://COSIGN_PRIVATE_KEY "{}@${DIGEST}"
         env:
           TAGS: ${{ steps.meta.outputs.tags }}
           COSIGN_PRIVATE_KEY: ${{secrets.COSIGN_PRIVATE_KEY}}