⚡Dockerize a Flask App | 3 Real-World Mistakes Debugged

DevOps Labs — Hands-On Dockerfile Debugging That Engineers Must Know

Author

Arbaz M
September 16, 2025

🎯 Why This Matters

In Episode 1, we ran our very first container. Fun, right?
But here’s the truth: real DevOps work doesn’t stop there.

Every engineer who writes their first Dockerfile hits the same wall:

  • Container runs, but app isn’t reachable.

  • Image builds, but CI/CD pipelines crawl.

  • “It works on my laptop” … but fails in production.

This episode is about turning those mistakes into production-grade lessons.

▶️ What You’ll Learn in This Video

In this step-by-step hands-on demo, I cover: https://youtu.be/PaUmQzT0ocs

📌 Dockerfile Basics

  • What a Dockerfile actually does

  • Why it’s like a shell script, but more powerful

  • Repeatability & portability: same image runs everywhere

📌 Hands-On Setup

  • Writing a simple Flask app + requirements.txt

  • First Dockerfile attempt

  • Building and running the container

📌 Debugging 3 Real Mistakes

  1. Missing Port Mapping → Container runs but browser can’t connect.

  2. Missing EXPOSE → Breaks automation in Compose/K8s.

  3. Bad Caching Strategy → Tiny code change forces full rebuilds.

📌 Best Practices You’ll Apply

  • Always copy requirements.txt first for caching

  • Always publish ports (-p) & document with EXPOSE

  • Use .dockerignore to keep images lean

  • Tag images properly — never rely on latest in production

👉 Watch the full video here: [YouTube Link]
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🛠 Takeaway Example Command

❓ How do you build and run a Docker image for your Flask app?

✅ Answer: 

docker build -t flask-app:v1 .
docker run -d --name flask-v1 -p 5000:5000 flask-app:v1

💡 Without -p 5000:5000, your container is alive but invisible.
This is Mistake #1 — one I’ve seen break real production deployments.

💡 Why This Guide Stands Out

Real-world focus → I don’t just “explain Docker,” I simulate mistakes.
Debug-driven → You’ll see docker ps, curl, and browser errors in action.
Production-ready → By the end, you’ll write Dockerfiles like an engineer, not a beginner.

This isn’t a Hello World. It’s Docker for real jobs.

👋 Final Note

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Every week I share real DevOps outages, interview prep, and hands-on labs you can reproduce — so you’ll never be caught off guard in production.