Fix: Flask Docker Container Exiting Due to Missing Gunicorn Entrypoint

Kishor Vaishnav

September 14, 2025

The Problem

When we Dockerized a Flask application, the container kept exiting immediately after startup.
Looking at the logs, we noticed that Flask’s built-in development server was being used by default. Since it’s not designed for production use, the container process would terminate unexpectedly.

This is a common mistake when containerizing Flask apps — forgetting to configure Gunicorn (a production-grade WSGI HTTP server for Python).

Why This Happens

By default, running flask run launches Flask’s development server.
In Docker, if the main process (PID 1) exits, the container stops.
Flask’s dev server isn’t stable in containerized environments.

That’s why we need Gunicorn as the container’s entrypoint.

The Solution ✅

We fixed the issue by updating the Dockerfile to use Gunicorn explicitly.

Updated Dockerfile

Here:

gunicorn is used instead of flask run. -b 0.0.0.0:8000 ensures the app listens on all interfaces inside the container.

app:app refers to the app object inside app.py.

Verifying the Fix

After rebuilding the image:

Output:

Now the Flask app stays running and stable inside Docker. 🎉

Key Takeaways

❌ Don’t rely on Flask’s development server in production or Docker.
✅ Always use Gunicorn (or uWSGI) for Flask apps.
✅ Ensure your CMD or ENTRYPOINT is set correctly in the Dockerfile.
✅ Keep containers alive by running a proper WSGI server process.

This simple change made our Flask Docker container production-ready and prevented unexpected shutdowns.

If you’re Dockerizing Flask apps, always check:

Are you using Gunicorn/uWSGI?
Is your entrypoint properly set?
Is the app bound to 0.0.0.0 instead of localhost?

These checks will save you hours of debugging. 🚀

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