If someone decides to not use `/.well-known/matrix/server` and only
relies on SRV records, then they would need to serve tcp/8448 using
a certificate for the base domain (not for the matrix) domain.

Until now, they could do that by giving the certificate to Synapse
and setting it terminate TLS. That makes swapping certificates
more annoying (Synapse requires a restart to re-read certificates),
so it's better if we can support it via matrix-nginx-proxy.

Mounting certificates (or any other file) into the matrix-nginx-proxy container
can be done with `matrix_nginx_proxy_container_additional_volumes`,
introduced in 96afbbb5a.