This guide provides a step-by-step walkthrough of containerizing a simple Flask web application and placing it behind an Nginx reverse proxy using Docker. It also covers orchestrating this multi-container setup with Docker Compose.

Prerequisites

Before starting, ensure you have the following:

• Host OS: Ubuntu 20.04+ (or similar Linux). (Provides the Linux environment required for installing Docker and the tools used in the lab.)
• User: sudo (root) privileges. (Necessary for installing system packages and managing Docker.)
• Tools:
  • pyenv (for Python versions). (A tool for managing multiple Python versions and virtual environments, useful for preparing your local development environment before containerization.)
  • Docker Engine & Docker CLI. (The core containerization platform.)
  • (Optional) Docker Compose plugin. (A tool for defining and running multi-container Docker applications, covered in the later part of the lab.)
• Network: If running inside a virtual machine (like VMware or VirtualBox), ensure you have forwarded host ports 81 and 8001 to your VM's IP address. (This allows you to access the application running inside the VM from your host machine's browser using those specific ports.)

1. Prepare Your Python/Flask Environment

We'll start by setting up a local Python environment using pyenv and installing Flask for initial testing.

• Activate your pyenv virtualenv:

  • (Switches your current shell session to use a specific Python version and its isolated environment managed by pyenv. We assume you have a Python 3.11.6 environment named py3_11_6 set up from a previous lab.)

  pyenv activate py3_11_6
  

• Install Flask:

  • (Installs the Flask web framework and its dependencies into the currently active virtual environment.)

  pip install flask
  

• Verify Flask version:

  • (Runs a small Python script directly in the shell to confirm that Flask was installed successfully and to check its version.)

  python - <<EOF
  from importlib.metadata import version
  print("Flask version:", version("flask"))
  EOF
  

• Deactivate when done:

  • (Exits the virtual environment, returning your shell to your system's default Python.)

  deactivate
  

2. Write & Run a Basic Flask App

Now, we'll create a simple "Hello World" Flask application and test it locally outside of Docker.

• Create your project folder:

  • (Creates a new directory named myapp for your project and navigates into it.)

  mkdir myapp && cd myapp
  

• Create main.py:

  • (Creates the main Python file for your Flask application. It defines a simple Flask app (app = Flask(__name__)), a route (@app.route('/')) that returns "hello world!" for the root URL, and configures the app to run on all network interfaces (host='0.0.0.0') on port 8001 when the script is executed directly.)

  # Create a file named main.py with the following content:
  from flask import Flask
  
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return 'hello world!'
  
  if __name__ == '__main__':
      # Run the app, listening on all interfaces (0.0.0.0) on port 8001
      app.run(host='0.0.0.0', port=8001)
  

• Run locally: (Activates the virtual environment where Flask was installed, then runs the main.py script, starting the Flask development server.)

  pyenv activate py3_11_6
  python main.py
  

  → Visit http://127.0.0.1:8001 in your browser to confirm the "hello world!" message. Stop the server with Ctrl+C, then

  deactivate