Introduction

"It works on my machine" is the classic developer problem. Your app works locally but fails on a server, because the server has a different version of Node, a missing library, or a different operating system setup.

Docker solves this by packaging the app together with its runtime, libraries, and settings into a container — a sealed box that runs the same way wherever Docker runs. If it works in the container on your laptop, it works in the same container on the server.

Image vs container: the one distinction that matters

An image is the blueprint: a read-only template that says "start from this base, install these things, run this command." A container is a running instance of an image — the blueprint brought to life.

One image can produce many containers, just as one class in programming can produce many objects. You build an image once, then start, stop, and delete containers from it as needed.…