Docker for Rails Developers

Reaching the App: Publishing Ports

As we know, containers are isolated sandboxes. How is it that we’re able to reach our app by visiting http://localhost:3000 on our local machine?

The truth is, containers wouldn’t be very useful if there was no way of reaching them from outside of Docker. The whole point of a web server, for example, is that it’s accessible to people making requests.

Although, by default, a container can only be accessed from within the Docker network it’s connected to (more on this in ​How Containers Can Talk to Each Other​), we can make it accessible externally by publishing one or more ports with docker run’s -p option.

In our command, we specified -p 3000:3000; this publishes the container’s port 3000 (the second number) ...