Network
When you run a compose stack, docker sets up mini-network so the containers can communicate with to each other using web protocols.
Ports#
Services may include a ports section that maps a container's port to the host machine so it may be accessed over localhost
DNS resolution#
Services may contact each other using their host name, which automatically resolves to the container's ip address.
See the Dashboard Example to see how this is utilized to connect to a local database.