> The various internal networks offer isolation of containers from the rest of the network stack. If I have a rails app in a docker compose configuration with PG, Redis, Elasticsearch, etc, in a development environment, most of the time I don't want the service/accessory containers anywhere near anything other than the app's container.

Which can be achieved with normal routed networks with firewall on the “router” just fine, even on IPv4. Each container can have its own IP address, its own hostname and listen on a standard port, all reachable from containers and from the main OS of the workstation.