🐳 Docker Debugging Deep Dive: Fixing Port Conflicts, Volumes & Networking in a Django + MySQL + NGINX Setup

Recently, I ran into a Docker-related issue that looked deceptively simple — but turned into a valuable deep-dive into container networking, volume persistence, and real-world troubleshooting.

Here’s a breakdown of what happened, how I debugged it, and what I learned.

---

🧩 The Stack

I was containerizing a basic full-stack app using:

• Django (backend API)

• MySQL (relational database)

• NGINX (reverse proxy)

The app was orchestrated using Docker Compose, with each service defined as a container.

---

❌ The Problem

When I spun up my containers, everything seemed to build just fine — until the deployment hit a wall with this error:

bashCopyEditError starting userland proxy: listen tcp4 0.0.0.0:80: bind: address already in use

In short: NGINX was trying to bind to port 80, but something else was already using it.

---

🛠️ The Fixes — Step by Step

1. 🔍 Solving the Port Conflict

This was the first and most immediate blocker. I found that my host machine already had a service (probably Apache or another NGINX instance) using port 80.

✅ Solution:

• Either stop the conflicting service, or

• Change the NGINX container’s exposed port in the docker-compose.yml:

yamlCopyEditports:
  - "8080:80"  # Now maps container port 80 to host port 8080

---

2. 🧹 Cleaning Up Residual MySQL Data

Even after restarting containers, MySQL was acting strangely — failing on startup with data-related errors.

✅ Solution:

• Removed the old volume storing MySQL data.

• Re-created the volume externally to decouple lifecycle from containers.

bashCopyEditdocker volume rm my_mysql_data
docker volume create my_mysql_data

Then updated docker-compose.yml:

yamlCopyEditvolumes:
  - my_mysql_data:/var/lib/mysql

---

3. 🌐 Creating a Custom Docker Bridge Network

To ensure all services could talk to each other reliably (especially NGINX to Django, and Django to MySQL), I created an external bridge network.

bashCopyEditdocker network create notes-app-nw

Then added this to the Compose file:

yamlCopyEditnetworks:
  default:
    external:
      name: notes-app-nw

This ensured all containers shared a common network with static naming.

---

🧪 Final Result

After these changes, I re-ran:

bashCopyEditdocker-compose up --build

And this time — everything just worked. ✅

• MySQL initialized with a fresh schema.

• Django connected cleanly to the database.

• NGINX reverse-proxied requests to Django on the internal network.

---

💡 Key Takeaways

• Port 80 errors are common — always double-check what’s running on your host.

• Volumes persist across container restarts — don’t forget to clean or isolate them when debugging.

• Custom Docker networks make multi-container setups more predictable and secure.

• Docker is powerful but requires systems thinking — it’s not just about code, it’s about configuration and infrastructure.

---

🧠 Final Thoughts

This was a great exercise in real-world Docker troubleshooting. It reminded me that debugging container issues isn’t just about error messages — it’s about understanding how your services interact, persist data, and expose themselves to the outside world.

If you’ve faced a similar issue — or have tips for handling Docker networking and volumes — drop a comment or share your experience!