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

😞

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! 🤘

Docker networking is used by docker-compose under the hood to transparently add all your compose services to a common network.