How to Containerize a Legacy Web Application

Overview

This guide teaches containerizing existing web applications with Docker. You'll create a Dockerfile, build optimized images, and deploy with Docker Compose — enabling reliable, scalable deployments.

Prerequisites

Docker installed on your development machine
Your web application source code (Node.js, Python, PHP, etc.)
Basic familiarity with your application's dependencies

Step 1: Create a Dockerfile

Create a Dockerfile in your application's root directory:

# Use official base image
FROM node:20-alpine

# Set working directory
WORKDIR /app

# Copy package files
COPY package*.json ./

# Install dependencies
RUN npm install --only=production

# Copy application code
COPY . .

# Expose port
EXPOSE 3000

# Health check
HEALTHCHECK --interval=30s CMD curl -f http://localhost:3000/health || exit 1

# Start application
CMD ["npm", "start"]

Step 2: Build the Docker Image

docker build -t myapp:1.0 .

Test locally:

docker run -p 3000:3000 myapp:1.0

Step 3: Create Docker Compose

Create docker-compose.yml for multi-container orchestration:

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
      - DATABASE_URL=postgresql://user:password@db:5432/app
    depends_on:
      - db
    restart: always
    volumes:
      - ./logs:/app/logs

  db:
    image: postgres:15-alpine
    environment:
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=password
      - POSTGRES_DB=app
    volumes:
      - db_data:/var/lib/postgresql/data
    restart: always

  nginx:
    image: nginx:alpine
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
      - /etc/letsencrypt:/etc/letsencrypt
    depends_on:
      - web
    restart: always

volumes:
  db_data:

Step 4: Deploy to Production

docker-compose up -d

Verify all services running:

docker-compose ps

Step 5: SSL/HTTPS Configuration

Install Certbot on the host and mount certificates into the nginx container:

sudo certbot certonly --standalone -d yourdomain.com

Next Steps

Implement health checks and monitoring
Set up automated backups for database containers
Deploy to production servers with Compose
Implement container logging and monitoring

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