How to Deploy Flask Using Docker: Step-by-Step Guide

To deploy a Flask app using Docker, create a Dockerfile that sets up Python, copies your app, installs dependencies, and runs the app. Then build the Docker image with docker build and run it using docker run to start your Flask app inside a container.

Syntax

A typical Dockerfile for Flask deployment includes these parts:

• Base image: Starts with a Python image.
• Working directory: Sets where files go inside the container.
• Copy files: Moves your Flask app code into the container.
• Install dependencies: Uses pip to install required packages.
• Expose port: Opens the port Flask listens on.
• Run command: Starts the Flask app with python.

dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE 5000
CMD ["python", "app.py"]

Example

This example shows a simple Flask app deployed with Docker. It includes the Flask app code, a requirements.txt file, and a Dockerfile. You build the image and run the container to serve the app on port 5000.

plaintext

# app.py
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "Hello from Flask in Docker!"

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

# requirements.txt
flask

# Dockerfile
FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
EXPOSE 5000
CMD ["python", "app.py"]

# Commands to build and run:
# docker build -t flask-docker-app .
# docker run -p 5000:5000 flask-docker-app

Output

When you visit http://localhost:5000 in your browser, you will see: Hello from Flask in Docker!

Common Pitfalls

Common mistakes when deploying Flask with Docker include:

• Not setting host='0.0.0.0' in app.run(), so the app is not accessible outside the container.
• Forgetting to expose the correct port in the Dockerfile or map ports with docker run -p.
• Not copying all necessary files or missing dependencies in requirements.txt.
• Using the default Flask debug server in production (consider using a production server like Gunicorn).

python

# Wrong: app.py without host set
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "Hello"

if __name__ == '__main__':
    app.run()  # Defaults to localhost only

# Right: app.py with host set
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "Hello"

if __name__ == '__main__':
    app.run(host='0.0.0.0')  # Accessible outside container

Quick Reference

Tips for deploying Flask with Docker:

• Always set host='0.0.0.0' in app.run().
• Use a slim Python base image for smaller containers.
• Keep dependencies in requirements.txt.
• Expose and map the correct port (usually 5000).
• Build images with docker build -t your-app-name ..
• Run containers with docker run -p 5000:5000 your-app-name.

Key Takeaways

Create a Dockerfile that installs dependencies and runs your Flask app with host set to 0.0.0.0.

Build your Docker image using docker build and run it with docker run mapping the ports.

Expose the Flask port in the Dockerfile and map it correctly to access the app from outside.

Include all necessary files and dependencies to avoid runtime errors.

Avoid using Flask's default server in production; consider production-ready servers for real deployments.