In-Class Exercise - Docker Container Debugging Challenge¶

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The Challenge¶

A web application container is crashing immediately after startup, and you need to debug it using Docker tools and techniques. Your mission: identify the root cause and fix the issue without modifying the original Dockerfile until you’ve diagnosed the problem.

Scenario¶

You’ve inherited a Python Flask application that worked yesterday but now fails to start. The container exits with code 1, and you need to investigate:

Why is the container crashing?
What files or configurations are missing?
How can you inspect a container that won’t stay running?
Can you fix it interactively before updating the Dockerfile?

Task Checklist¶

Build the provided broken Docker image
Attempt to run it and observe the failure
Use Docker debugging techniques to inspect the crashed container
Override the entrypoint to keep the container alive for investigation
Explore the filesystem and identify the missing configuration file
Fix the issue interactively and verify the application works
Update the Dockerfile with the permanent fix
Document your debugging process

Acceptance Criteria¶

Identify the root cause without looking at the solution first
Successfully get the application running using debugging techniques
Demonstrate at least two different debugging methods (e.g., docker logs, entrypoint override, docker exec)
Apply a permanent fix to the Dockerfile

Tips¶

docker logs <container> shows output even from crashed containers
docker run --entrypoint /bin/sh can override the startup command
docker exec requires a running container, but docker run -it with an overridden entrypoint keeps it alive
The --rm flag is helpful during debugging to avoid container clutter
Use docker inspect to see container configuration and state

Solution

Broken Application Files¶

`app.py`¶

from flask import Flask
import json

app = Flask(__name__)

# Load configuration
with open('/app/config.json', 'r') as f:
    config = json.load(f)

@app.route('/')
def hello():
    return f"Hello from {config['app_name']}! Running on port {config['port']}\n"

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=config['port'])

`requirements.txt`¶

Flask==3.0.0

`Dockerfile` (Broken Version)¶

# syntax=docker/dockerfile:1
FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY app.py .

# Missing: config.json is NOT copied!

EXPOSE 5000

CMD ["python", "app.py"]

Step-by-Step Debugging Solution¶

1. Build the Broken Image¶

docker build -t broken-flask-app .

2. Attempt to Run (This Will Fail)¶

docker run --rm --name flask-debug -p 5000:5000 broken-flask-app

Observation: Container exits immediately with error.

3. Check the Logs¶

# If using --rm, run without it first to preserve the container
docker run --name flask-debug -p 5000:5000 broken-flask-app

# In another terminal
docker logs flask-debug

Expected Output:

Traceback (most recent call last):
  File "/app/app.py", line 6, in <module>
    with open('/app/config.json', 'r') as f:
FileNotFoundError: [Errno 2] No such file or directory: '/app/config.json'

Diagnosis: Missing config.json file!

4. Debug by Overriding the Entrypoint¶

docker run --rm -it --entrypoint /bin/sh broken-flask-app

Inside the container:

ls -la /app/
# Shows: app.py, requirements.txt - but NO config.json!

# Verify Python can't find it
python -c "open('/app/config.json')"
# Error: No such file or directory

5. Create the Missing Configuration File¶

Create config.json locally:

{
  "app_name": "Flask Debug Demo",
  "port": 5000
}

6. Fix Interactively with Volume Mount¶

# Mount the config file into the container for testing
docker run --rm -p 5000:5000 \
  -v $(pwd)/config.json:/app/config.json \
  broken-flask-app

Test the application:

curl http://localhost:5000