I was wrong. It's not the container name but I forgot simply to give the
network the very same name as specified in the docker-compose.yml of the
frontend
After we proxy API requests through the server-side-rendered frontend
there is no need to use xip.io anymore. However, if you "join" services
to a named network at arbitrary times, the DNS of docker-compose only
works if you assign a name to the container. Thus I left `backend` as a
container name.
