So far we've just been running containers with random tags that I chose. If you run docker run -it node the tag implicitly is using the latest tag. When you say docker run -it node, it's the same as saying docker run -it node:latest. The :latest is the tag. This allows you to run different versions of the same container, just like you can install React version 15 or React version 16: some times you don't want the latest. Let's say you have a legacy application at your job and it depends on running on Node.js 8 (update your app, Node.js is already past end-of-life) then you can say

docker run -it node:8 bash

Once in the shell, run node --version and you'll see the Node.js version is 8..! Neat! This is helpful because now we can fix our Node.js version to the one our app expects. Hop back over to the Docker Hub page for the node container. Take a look at all the version of the node container you can download. Let's try another one.

docker run node:12-alpine cat /etc/issue

You'll see this is running an entirely different OS all together: Alpine! Alpine Linux is a very, very tiny distro of Linux made for containers and specifically because it is tiny. Alpine containers are bare bones: if you want anything in them, you're going to have to do it yourself. This is in opposition to the Ubuntu and Debian containers: they ship the kitchen sink with them which is both convenient and much bigger in size. Alpine images are about five megabytes whereas Ubuntu is close to two hundred megabytes. As you can imagine, this can make a difference in how fast you can deploy and can cost significantly less in terms of storage and network traffic. It's also in general better to have less unnecessary things in your containers: less is more in terms of security. If an attacker tries to execute a Python exploit on your container but your container doesn't have Python then their attack won't work.

We'll get more into how to ship containers to production but I'll leave you with this pro-tip: have a development container which has all the bells, whistles, debugging tools, etc. that you need. Then have a production container that's minimalist as possibly can be. You'll get the best of both worlds.