Use examples that work with multiple tags (#155)

Signed-off-by: Jürgen Kreileder <jk@blackdown.de>

README.md

@@ -137,7 +137,11 @@ jobs:
       # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
       - name: Sign image with a key
         run: |
-          cosign sign --yes --key env://COSIGN_PRIVATE_KEY "${TAGS}@${DIGEST}"
+          images=""
+          for tag in ${TAGS}; do
+            images+="${tag}@${DIGEST} "
+          done
+          cosign sign --yes --key env://COSIGN_PRIVATE_KEY ${images}
         env:
           TAGS: ${{ steps.docker_meta.outputs.tags }}
           COSIGN_PRIVATE_KEY: ${{ secrets.COSIGN_PRIVATE_KEY }}
@@ -148,7 +152,12 @@ jobs:
         env:
           DIGEST: ${{ steps.build-and-push.outputs.digest }}
           TAGS: ${{ steps.docker_meta.outputs.tags }}
-        run: cosign sign --yes "${TAGS}@${DIGEST}"
+        run: |
+          images=""
+          for tag in ${TAGS}; do
+            images+="${tag}@${DIGEST} "
+          done
+          cosign sign --yes ${images}
 ```
 
 ### Optional Inputs