Docker lets you package apps and dependencies into containersthat run the same everywhere. This guide covers the essentials to get you started fast.

1. What is Docker?

Docker is a containerization platform. Containers share the OS kernel, so they are lighter and faster than full virtual machines.

• Portability: run the same in any environment.
• Consistency: dev, test and prod match.
• Isolation: apps don't conflict.
• Efficiency: fewer resources than VMs.

2. Core concepts

• Image: read-only template with app and dependencies.
• Container: running instance of an image.
• Dockerfile: recipe to build images.

3. Install Docker

Windows and macOS: install Docker Desktop from the official site.

Linux (Ubuntu/Debian):

sudo apt update
sudo apt install apt-transport-https ca-certificates curl software-properties-common
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
sudo apt update
sudo apt install docker-ce
docker --version

4. Essential commands

Images:

docker pull nginx
docker images
docker rmi nginx

Containers:

docker run -d -p 8080:80 nginx
docker ps
docker ps -a
docker stop <container_id>
docker rm <container_id>
docker exec -it <container_id> sh

5. Your first Docker project (Node.js)

Folder:

mkdir my-first-docker
cd my-first-docker

package.json:

{
  "name": "my-first-docker",
  "version": "1.0.0",
  "scripts": { "start": "node server.js" },
  "dependencies": { "express": "^4.18.0" }
}

server.js:

const express = require("express");
const app = express();
const PORT = process.env.PORT || 3000;

app.get("/", (req, res) => {
  res.send("<h1>My first Docker app!</h1>");
});

app.listen(PORT, () => console.log("Running on port " + PORT));

Dockerfile:

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Build and run:

docker build -t my-app .
docker run -d -p 3000:3000 --name my-container my-app
curl http://localhost:3000

.dockerignore:

node_modules
npm-debug.log
.git
.DS_Store

6. Docker Compose

For multiple services, create a docker-compose.yml:

version: "3.8"
services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
    depends_on:
      - db
  db:
    image: postgres:13
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
    volumes:
      - postgres_data:/var/lib/postgresql/data
volumes:
  postgres_data:

Run:

docker compose up -d
docker compose down

7. Best practices

• Prefer official images.
• Use .dockerignore to reduce image size.
• Combine layers where possible.
• Use multi-stage builds for larger apps.
• Avoid running as root.
• Add health checks.

Next steps

Explore image optimization, CI/CD with Docker, and orchestration with Docker Swarm or Kubernetes.