Docker Essentials

Quick reference guide for Docker fundamentals.

Basic Concepts

What is Docker?

Docker is a platform for developing, shipping, and running applications in containers.

Key Benefits:

Consistent environments

Isolation

Portability

Resource efficiency

Installation

Linux

curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh

Verify Installation

docker --version
docker run hello-world

Images

Pull an Image

docker pull nginx
docker pull node:18-alpine

List Images

docker images
docker images -a

Remove Image

docker rmi nginx
docker rmi $(docker images -q)  # Remove all

Build Image

docker build -t myapp:1.0 .
docker build -t myapp:latest --no-cache .

Containers

Run Container

# Basic run (foreground mode - blocks terminal)
docker run nginx

# Detached mode (runs in background)
docker run -d nginx

# With port mapping (host:container)
# Access nginx on http://localhost:8080
docker run -d -p 8080:80 nginx

# With custom name (easier to reference later)
docker run -d --name my-nginx nginx

# With environment variables
docker run -d -e NODE_ENV=production node-app

# Complete example (name + port + env)
docker run -d \
  --name my-app \
  -p 3000:3000 \
  -e NODE_ENV=production \
  node-app

List Containers

docker ps              # Running containers
docker ps -a           # All containers
docker ps -q           # Container IDs only

Stop/Start Container

docker stop my-nginx
docker start my-nginx
docker restart my-nginx

Remove Container

docker rm my-nginx
docker rm -f my-nginx  # Force remove running
docker rm $(docker ps -aq)  # Remove all stopped

Container Logs

docker logs my-nginx
docker logs -f my-nginx  # Follow logs
docker logs --tail 100 my-nginx

Execute Commands

docker exec my-nginx ls /usr/share/nginx/html
docker exec -it my-nginx bash
docker exec -it my-nginx sh

Dockerfile

Basic Structure

Step-by-step Dockerfile explanation:

# 1. Choose a base image (lightweight Alpine Linux variant)
FROM node:18-alpine

# 2. Set working directory inside container
WORKDIR /app

# 3. Copy dependency files first (for better caching)
COPY package*.json ./

# 4. Install dependencies
RUN npm install

# 5. Copy application source code
COPY . .

# 6. Document which port the app uses
EXPOSE 3000

# 7. Define the command to run the app
CMD ["npm", "start"]

Why this order matters:

Dependencies change less often than code

Docker caches layers → faster rebuilds

Only changed layers are rebuilt

Multi-stage Build

# Build stage
FROM node:18-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build

# Production stage
FROM node:18-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY package*.json ./
RUN npm install --production
EXPOSE 3000
CMD ["node", "dist/main.js"]

Docker Compose

docker-compose.yml

version: '3.8'

services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
    depends_on:
      - db
    volumes:
      - ./src:/app/src

  db:
    image: postgres:15
    environment:
      - POSTGRES_PASSWORD=secret
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Compose Commands

# Start services
docker-compose up
docker-compose up -d

# Stop services
docker-compose down
docker-compose down -v  # Remove volumes

# View logs
docker-compose logs
docker-compose logs -f web

# Rebuild
docker-compose build
docker-compose up --build

Volumes

Create Volume

docker volume create mydata
docker volume ls
docker volume inspect mydata

Use Volume

docker run -v mydata:/data nginx
docker run -v $(pwd):/app node-app  # Bind mount

Remove Volume

docker volume rm mydata
docker volume prune  # Remove unused

Networks

Create Network

docker network create mynet
docker network ls
docker network inspect mynet

Connect Container

docker run -d --network mynet --name web nginx
docker network connect mynet db

Best Practices

1. Use Official Images

FROM node:18-alpine  # Official + minimal

2. Multi-stage Builds

Reduce final image size by copying only necessary files.

3. Layer Caching

# Copy dependencies first
COPY package*.json ./
RUN npm install

# Then copy source
COPY . .

4. .dockerignore

node_modules
.git
.env
*.md

5. Non-root User

RUN addgroup -S appgroup && adduser -S appuser -G appgroup
USER appuser

6. Health Checks

HEALTHCHECK --interval=30s --timeout=3s \
  CMD wget --quiet --tries=1 --spider http://localhost:3000/health || exit 1

Cleanup Commands

# Remove stopped containers
docker container prune

# Remove unused images
docker image prune

# Remove unused volumes
docker volume prune

# Remove everything
docker system prune -a

Common Issues

Port Already in Use

# Find process using port
lsof -i :8080
# Kill process or use different port

Permission Denied

sudo usermod -aG docker $USER
# Logout and login again

Out of Disk Space

docker system prune -a --volumes

Resources

Docker Documentation

Docker Hub

Best Practices