Docker networking

Most applications these days do not run in isolation but need to communicate with other systems over the network. If we want to run a website, web service, database, or a cache server inside a Docker container, then we need to understand at least the basics of Docker networking.

Running services

Let's start with a simple example, and run a Tomcat server directly from Docker Hub:

$ docker run -d tomcat

Tomcat is a web application server whose user interface can be accessed by the port 8080. Therefore, if we installed Tomcat on our machine, we could browse it at http://localhost:8080.

In our case, however, Tomcat is running inside the Docker container. We started it the same way we did with the first Hello World example. We can see it's running:

$ docker ps
CONTAINER ID IMAGE  COMMAND           STATUS            PORTS    NAMES
d51ad8634fac tomcat "catalina.sh run" Up About a minute 8080/tcp jovial_kare

Since it's run as a daemon (with the -d option), we don't see the logs in the...