Docker Introduction

Docker packages an application and everything it needs — code, runtime, libraries, system tools — into a container: a lightweight, isolated unit that runs identically on any machine.

The problem it solves

It ends the classic “it works on my machine” problem. The container bundles the exact environment, so what runs on your laptop runs the same in CI and in production — no more missing dependencies or version mismatches.

Two key ideas

• An image is a read-only template — your app plus its dependencies, built once.
• A container is a running instance of an image — you can start many from one image.

The mental model: an image is like a class, a container like an object created from it.

Containers vs virtual machines

Containers share the host operating system's kernel, so they're far lighter than VMs — you can run dozens on one machine.

Why teams use it

• Consistency across dev, test, and production.
• Fast, reproducible environments — spin up a database in seconds.
• The foundation of modern CI/CD and Kubernetes deployments.