The website for https://integration.wikimedia.org is in integration/docroot.git which is cloned at /srv
/srv belongs to root:root and also is a place for different materials such as the jenkins build history or zuul git repositories.
I had to manually hack /srv to be owned by jenkins-slave. It should be made back to root:root AND the .git repo should be cloned one level deeper instead of directly in /srv. A good candidate would be /srv/docroot.