Most (all?) of our Matrix services are running in the `matrix` network,
so they were safe -- not accessible from Coturn to begin with.
Isolating Coturn into its own network is a security improvement
for people who were starting other services in the default
Docker network. Those services were potentially reachable over the
private Docker network from Coturn.
Discussed in #120 (Github Pull Request)
24cf27c60c