Docker Image Layers Explained - Why Containers Are Lightning Fast

Docker Image Layers Breakdown:

Layer 1: FROM node:24

dockerfile

FROM node:16

What it provides:

- Base operating system (usually Debian/Alpine Linux)

- Node.js runtime environment

- npm package manager

- System libraries and dependencies

---

Layer 2: RUN apt-get update

dockerfile

RUN apt-get update

What it does:

- Updates package repository indexes

- Ensures latest package versions are available

- Creates a new layer with updated package lists

---

Layer 3: RUN npm install

dockerfile

RUN npm install

```

What it does:

- Installs all dependencies from package.json

- Downloads node_modules

- Adds application dependencies to the image

---

Layer 4: COPY . /app

dockerfile

COPY . /app

```

What it does:

- Copies your application source code

- Adds configuration files

- Includes all your custom files into the image

---

🎯 Key Benefits of This Layering:

Storage Efficiency:

Container A (Node App)    Container B (Node App)

├── App Code              ├── Different App Code

├── npm install           ├── npm install           ← SHARED LAYER

└── FROM node:16          └── FROM node:16          ← SHARED LAYER

```

Build Speed:

- If you change only your code (Layer 4), Layers 1-3 are **cached**

- Faster rebuilds using existing layers

- No need to re-download dependencies

Real Dockerfile Example:

```dockerfile

# LAYER 1: Base image (shared across containers)

FROM node:16

# LAYER 2: System updates

RUN apt-get update && apt-get install -y curl

# LAYER 3: Dependencies

COPY package*.json ./

RUN npm install

# LAYER 4: Application code

COPY . .

# LAYER 5: Runtime configuration

CMD ["node", "server.js"]

```

Each `RUN`, `COPY`, or `ADD` command creates a new **read-only layer** in the Docker image!