How Can You Bottle a Web App? Let’s Dockerize a Flask App and See the Magic!

3 min readMay 13, 2025

Have you ever dreamed of packaging your web app like a lunchbox — ready to go, easy to open, and works anywhere? That’s exactly what Docker helps us do!

In this project, I took a simple Flask app and learned how to wrap it neatly inside a Docker container. It felt like turning my tiny code project into a portable superhero. 🦸‍♂️💻

Let me walk you through what I learned, step-by-step, as a complete beginner.

🐍 Step 1: Meet Flask — The Friendly Web Framework

Flask is a lightweight web framework in Python. It’s super easy to use when you want to build small web apps or APIs. Here’s a tiny Flask app:

from flask import Flask
app = Flask(__name__)

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

We saved this as app.py. Simple, right?

🐳 Step 2: What is Docker and Why Should I Care?

Docker is like a magic box. You put your app inside it, close the lid, and now your app can run anywhere, no matter what OS or software is outside. 🧊

This makes sharing and deploying apps super easy — whether it’s your friend’s laptop, your college server, or the cloud.

📦 Step 3: Writing the Dockerfile (a Recipe for the Magic Box)

Here’s how I told Docker how to build the container:

# Use an official Python image
FROM python:3.9-slim

# Set working directory
WORKDIR /app

# Copy app files
COPY app.py requirements.txt ./

# Install Flask
RUN pip install -r requirements.txt

# Set environment variable (optional but cool!)
ENV FLASK_ENV=development

# Tell Docker how to run the app
CMD ["python", "app.py"]

I saved this as Dockerfile—no file extension!

The line ENV FLASK_ENV=development helped me pass configuration into the container. It’s like setting the app’s mood from outside!

🚀 Step 4: Build and Run the Flask Container

I used just two commands:

docker build -t flask-docker-app .
docker run -p 5000:5000 flask-docker-app

Boom! 🎉 My app was now running on http://localhost:5000, and it was inside a container. No Flask installed on my machine—just Docker!

⚙️ CMD vs ENTRYPOINT — What’s the Difference?

This part confused me at first, but here’s what I figured out:

CMD is like the default command. You can override it when you run the container.
ENTRYPOINT is the main command. It sticks no matter what.

Example:

ENTRYPOINT ["python", "app.py"]
CMD ["--port=8000"]

Here, CMD adds extra options to the ENTRYPOINT command.

💡 Bonus: Passing Input and Playing with Behavior

I tried passing command-line inputs like this:

docker run flask-docker-app --port=8000

Also, I experimented with changing the ENV variable to switch between dev and production modes. It's fun seeing how flexible and powerful containers can be!

🔚 Wrapping It Up: Why This Matters

Before this project, Docker felt like a tech buzzword. But now? I actually get it. 😎

It’s not just about making things “cool” or “modern.” Docker makes your app portable, consistent, and deployment-ready. Whether you’re a solo dev, a student, or working in a team — it just makes life easier.

So yes, you can bottle up a web app — and I just did it with Flask and Docker!