What is Docker?

Docker is a tool that lets you package an application along with everything it needs (code, libraries, settings, runtime) into one unit called a container.

Docker = "Pack once, run anywhere"

Problem Before Docker

The classic situation:

• App works on your laptop
• Does not work on your friend's laptop
• Does not work on the server

Common reasons:

• Different OS versions
• Different Java / Node / Python versions
• Missing libraries
• Different environment configs

This is the famous "It works on my machine" problem.

How Docker Solves This

Docker puts everything needed inside a container:

• Application code
• Runtime (Java, Node, Python, etc.)