We work inside Docker containers all the time, as our development environment.
As a result, we sometimes want to spin up sibling containers and connect to them.
Example:
In the following example, I want to connect my current container, myproject_dev-env_run_1, started using docker-compose to the myapp container, which was started using the docker command from within the dev-env container:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
874a13f65926 myapp "httpd-foreground" 11 minutes ago Up 11 minutes 0.0.0.0:8080->80/tcp myapp
034a8606b4b0 myproject-dev-env:latest "bash" 37 minutes ago Up 37 minutes myproject_dev-env_run_1
curl http://myapp
curl: (6) Could not resolve host: myapp
š
However, I can connect the myapp container to my current container network
$ docker network ls
NETWORK ID NAME DRIVER SCOPE
...
76a9193ca4b4 myproject_default bridge local
Connect the myapp container to my current network
docker network connect 76a9193ca4b4 874a13f65926
# curl http://myapp
<html>
<head>
<Title>Hello World</title>
</head>
<body>
Hello World!
</body>
</html>
BOOM! š¤
Docker networking is used by docker-compose under the hood to transparently add all your compose services to a common network.