Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3701

We can restore support for it by making a non-/ path prefix trigger a
self-build with a modified build config, so that a Cinny version with
path-prefix support would be produced. Traefik label changes may also be
required.

For now, we're not doing all that work, but simply dropping support for path
prefixes for Cinny.