fix: signing also needs to be lowercase 😭 (#57)

.github/workflows/build.yml

@@ -166,7 +166,7 @@ jobs:
       - name: Sign container image
         if: github.event_name != 'pull_request' && github.ref == format('refs/heads/{0}', github.event.repository.default_branch)
         run: |
-          IMAGE_FULL="${{ env.IMAGE_REGISTRY }}/${IMAGE_NAME}"
+          IMAGE_FULL="${{ steps.registry_case.outputs.lowercase }}/${IMAGE_NAME}"
           for tag in ${{ steps.metadata.outputs.tags }}; do
             cosign sign -y --key env://COSIGN_PRIVATE_KEY $IMAGE_FULL:$tag
           done