fail-proofing tag deletion

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

.gitea/workflows/docker.yaml

@@ -57,8 +57,11 @@ jobs:
         run: |
           set -x
           cat /tmp/old_tags | while read TAG; do
-            echo about to delete $TAG
             DIGEST=$(curl -u "${{ secrets.REGISTRY_USER }}:${{ secrets.REGISTRY_PASS }}" -sS -H 'Accept: application/vnd.docker.distribution.manifest.v2+json' -o /dev/null -w '%header{Docker-Content-Digest}' https://${{ secrets.REGISTRY_PATH }}/v2/umbrella/manifests/$TAG)
-            echo digest: $DIGEST
-            curl -u "${{ secrets.REGISTRY_USER }}:${{ secrets.REGISTRY_PASS }}" -sS -X DELETE https://${{ secrets.REGISTRY_PATH }}/v2/umbrella/manifests/$DIGEST
+            if [ -n "$DIGEST" ]; then
+              echo about to delete $TAG
+              curl -u "${{ secrets.REGISTRY_USER }}:${{ secrets.REGISTRY_PASS }}" -sS -X DELETE https://${{ secrets.REGISTRY_PATH }}/v2/umbrella/manifests/$DIGEST
+            else
+              echo failed to get digest for $TAG
+            fi
           done