fix: Replace invalid docker tag characters with underscore

.forgejo/workflows/images.yml

@@ -28,7 +28,8 @@ jobs:
           if [ "${{ github.ref_name }}" = "main" ] && [ "${{ github.event_name }}" = "push" ]; then
             echo "tag=latest" >> $GITHUB_OUTPUT
           else
-            echo "tag=${{ github.ref_name }}" >> $GITHUB_OUTPUT
+            # renovate creates sub branches with `/`; these break the tagging in the build process
+            echo "tag=${{ github.ref_name }}" | sed 's/[^a-zA-Z0-9._-]/_/g' >> $GITHUB_OUTPUT
           fi
 
       - name: Build web image