Docker — The Container Platform

Docker packages your application and all its dependencies into a container — a lightweight, portable, isolated unit that runs identically on any machine. No more "it works on my laptop."

ℹ️ What is a Container?

A container is not a virtual machine. It shares the host OS kernel but has its own isolated filesystem, process space, and network. This makes containers start in milliseconds and use 10-100× less memory than VMs.

How Docker Works

VM vs Container

	Virtual Machine	Docker Container

|---|---|---|

Start time	30 seconds – 5 minutes	Milliseconds
Size	GB (full OS)	MB (just app + deps)
Isolation	Full hardware virtualization	OS kernel shared
Performance	~10-20% overhead	Near-native
Use case	Full OS needed	App packaging & scaling

The Docker Build Workflow

Your First Dockerfile

dockerfile

# Start from official Node.js image
FROM node:20-alpine

# Set working directory inside container
WORKDIR /app

# Copy dependency files first (better layer caching)
COPY package*.json ./
RUN npm ci --only=production

# Copy source code
COPY . .

# Expose the port your app runs on
EXPOSE 3000

# Command to start the app
CMD ["node", "server.js"]

Essential Commands

bash

# Build an image from Dockerfile in current directory
docker build -t myapp:1.0 .

# Run a container
docker run -d -p 3000:3000 --name my-container myapp:1.0

# List running containers
docker ps

# View logs
docker logs -f my-container

# Stop and remove
docker stop my-container && docker rm my-container

# List images
docker images

💡 Key Concept: Images vs Containers

An Image is the blueprint (read-only). A Container is a running instance of an image. You can run many containers from one image — like a class and its objects in programming.

When to Use Docker

•Consistent environments — dev, staging, prod all run identical software

•Microservices — each service in its own container, independently deployable

•CI/CD pipelines — build once, run anywhere

•Local development — spin up databases, Redis, Kafka with one command

•Kubernetes — all K8s workloads are containers

Docker — Overview