Docker and Containerization Guide

This guide introduces Docker, explaining how to containerize applications for consistent development, testing, and production environments.

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1. What is Docker?

Docker is a platform that uses OS-level virtualization to deliver software in packages called containers. Containers are isolated from each other and bundle their own software, libraries, and configuration files.

Key Benefits

• Consistency: “It works on my machine” becomes “It works everywhere.”
• Isolation: Dependencies for one app won’t interfere with another.
• Efficiency: Containers share the host OS kernel, making them lightweight.

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2. Core Concepts

Component	Description
Dockerfile	A text file containing instructions to build a Docker Image.
Image	A read-only template used to create containers.
Container	A running instance of an image.
Docker Hub	A registry for sharing and finding Docker images.

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3. Containerizing a Flask App

Assume we have a Flask app in a directory with a requirements.txt.

Step 1: Create a Dockerfile

Create a file named Dockerfile (no extension) in your project root.

# 1. Use an official Python runtime as a parent image
FROM python:3.11-slim

# 2. Set the working directory in the container
WORKDIR /app

# 3. Copy requirements and install dependencies
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# 4. Copy the rest of the application code
COPY . .

# 5. Expose the port the app runs on
EXPOSE 5000

# 6. Define the command to run the app
CMD ["python", "app.py"]

Step 2: Build the Image

docker build -t flask-app-demo .

Step 3: Run the Container

docker run -p 5000:5000 flask-app-demo

The app is now accessible at http://localhost:5000.

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4. Multi-Container Apps with Docker Compose

Docker Compose is a tool for defining and running multi-container Docker applications (e.g., Flask + PostgreSQL).

Example docker-compose.yml

version: '3.8'

services:
  web:
    build: .
    ports:
      - "5000:5000"
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/mydb
    depends_on:
      - db

  db:
    image: postgres:15
    environment:
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=pass
      - POSTGRES_DB=mydb
    volumes:
      - postgres_data:/var/lib/postgresql/data

volumes:
  postgres_data:

Running the Stack

docker-compose up -d

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5. Common Commands

• docker ps: List running containers.
• docker images: List downloaded/built images.
• docker stop <id>: Stop a container.
• docker rm <id>: Remove a container.
• docker rmi <id>: Remove an image.
• docker exec -it <id> bash: Open a terminal inside a running container.

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See Also

• CI/CD Pipeline Guide - Automating Docker builds with GitHub Actions.
• Flask Production Workflow - Best practices for deploying Flask.