Docker: How do I debug localhost "Internal Server Error"?

Question

First time using Docker(version 19.03.5) and trying this tutorial
I'm stuck on step 2.3.4 Running an image
When I go to http://localhost:8888 I see
Internal Server Error The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

I updated Dockerfile to this to match my directory:

# our base image
FROM alpine:3.5

# Install python and pip
RUN apk add --update py2-pip

# install Python modules needed by the Python app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# copy files required for the app to run
COPY app.py .
COPY templates/index.html templates

# tell the port number the container should expose
EXPOSE 5000

# run the application
CMD ["python", "app.py"]

On my command line I have
C:\Users\user\docker\flask-app>docker run -p 8888:5000 --name flask-app 11111111/flask-app
* Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

When I visit the page I see on the prompt
172.17.0.1 - - [05/Jan/2020 07:14:34] "GET / HTTP/1.1" 500 -

I have this in my app.py

from flask import Flask, render_template
import random

app = Flask(__name__)

# list of cat images
images = [
   "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr05/15/9/anigif_enhanced-buzz-26388-1381844103-11.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr01/15/9/anigif_enhanced-buzz-31540-1381844535-8.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr05/15/9/anigif_enhanced-buzz-26390-1381844163-18.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr06/15/10/anigif_enhanced-buzz-1376-1381846217-0.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr03/15/9/anigif_enhanced-buzz-3391-1381844336-26.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr06/15/10/anigif_enhanced-buzz-29111-1381845968-0.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr03/15/9/anigif_enhanced-buzz-3409-1381844582-13.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr02/15/9/anigif_enhanced-buzz-19667-1381844937-10.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr05/15/9/anigif_enhanced-buzz-26358-1381845043-13.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr06/15/9/anigif_enhanced-buzz-18774-1381844645-6.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr06/15/9/anigif_enhanced-buzz-25158-1381844793-0.gif",
    "http://img.buzzfeed.com/buzzfeed-static/static/2013-10/enhanced/webdr03/15/10/anigif_enhanced-buzz-11980-1381846269-1.gif"
    ]

@app.route('/')
def index():
    url = random.choice(images)
    return render_template('index.html', url=url)

if __name__ == "__main__":
    app.run(host="0.0.0.0")

I can't figure out why my page isnt loading. Any help would be appreciated.

Note: I have WAMP installed and this might be conflicting but not sure if thats the case and/or how to fix it.

Answer 1

Flask might be unable to locate your templates. Try changing

COPY templates/index.html templates

to

COPY templates templates

to copy everything inside ./templates to <WORKDIR>/templates.

Using COPY templates/index.html templates will copy index.html as a file at path <WORKDIR>/templates, not copy it under that directory.

Answer 2

How to debug your Flask application in Docker:

1. Turn on Flask debugger by adding ENV FLASK_DEBUG=1 to your Dockerfile
2. Try to run the Flask app outside of Docker. It may be easier to set breakpoints inside your IDE (VSCode or PyCharm) and debug the app.
3. Try pdb to debug the app inside the container. It may be hard for a beginner, but in general, it's an essential skill. Check Debugging a Python Flask Application in a Docker Container for a step-by-step guide.