What is a Docker Image?
A Docker image is a read-only template containing everything needed to run an application — code, runtime, libraries, environment variables, and configuration files.
Image Structure
An image is made of LAYERS stacked on top of each other:
┌─────────────────────────┐
│ Your Application │ ← Layer 4: Your code
├─────────────────────────┤
│ Dependencies (npm) │ ← Layer 3: npm install
├─────────────────────────┤
│ Node.js Runtime │ ← Layer 2: apt-get install nodejs
├─────────────────────────┤
│ Ubuntu Base │ ← Layer 1: FROM ubuntu
└─────────────────────────┘
Each layer is cached — if nothing changes, Docker reuses it
This makes builds FAST after the first time
Image vs Container
Registries