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Welcome to this step-by-step guide on building, testing, and debugging your Flask application locally within a Docker container. By the end of this tutorial, you’ll be able to:
Containerize a Flask app using Docker
Identify and fix common configuration typos
Run and verify your application on a custom host port
Ensure you have Docker installed and Python 3.8+ on your local machine before you begin. Prerequisites
Python Flask application (app.py)
requirements.txt listing Flask (and any other dependencies)A Dockerfile to containerize your application
Project Structure ├── app.py ├── requirements.txt └── Dockerfile
1. Initial Dockerfile Below is our starting Dockerfile—note the typo in the CMD instruction that we’ll address later:
FROM python:3.8-slim-buster WORKDIR /app COPY requirements.txt requirements.txt RUN pip3 install -r requirements.txt COPY . # Note: the colon after --host is incorrect here CMD [ "python3" , "-m" , "flask" , "run" , "--host:0.0.0.0" ]
2. Building the Docker Image Open your terminal and execute:
docker build -t flask-docker-demo .
You should see output similar to:
[+] Building 2.0s ( 10/10 ) FINISHED = > [internal] load build definition from Dockerfile 0.0s = > [internal] load .dockerignore 0.0s = > [internal] load metadata for docker.io/library/python:3.8-slim-buster 0.0s = > [1/5] FROM docker.io/library/python:3.8-slim-buster@sha256:… 0.0s = > [2/5] WORKDIR /app 0.0s = > [3/5] COPY requirements.txt requirements.txt 0.0s = > [4/5] RUN pip3 install -r requirements.txt 0.0s = > [5/5] COPY . 0.0s = > exporting to image 0.0s = > = > writing image sha256:215f34… 0.0s = > = > naming to docker.io/flask-docker-demo 0.0s
Verify the image exists:
docker images | grep flask-docker-demo
3. Running the Container Try starting the container and mapping container port 5000 to host port 5000:
docker run -p 5000:5000 flask-docker-demo
Common Port Binding Error docker: Error response from daemon: Ports are not available: listen tcp 0.0.0.0:5000: bind: address already in use.
If you see this, remap the host port (e.g., to 5001):
docker run -p 5001:5000 flask-docker-demo
Flask Option Parsing Error Usage: python -m flask run [OPTIONS] Try 'python -m flask run --help' for help. Error: No such option: --host:0.0.0.0
This happens because Flask expects --host in the form --host=<address>, not --host:<address>.
4. Fixing the Dockerfile Update the CMD line to use = instead of ::
FROM python:3.8-slim-buster WORKDIR /app COPY requirements.txt requirements.txt RUN pip3 install -r requirements.txt COPY . # Fixed: use '=' instead of ':' CMD [ "python3" , "-m" , "flask" , "run" , "--host=0.0.0.0" ]
Rebuild the image:
docker build -t flask-docker-demo .
5. Running Successfully Start the container on host port 5001:
docker run -p 5001:5000 flask-docker-demo
You should see:
* Debug mode: off WARNING: This is a development server. Do not use it in a production deployment. * Running on all addresses ( 0.0.0.0 ) * Running on http://127.0.0.1:5000 * Running on http://172.17.0.2:5000 Press CTRL+C to quit
Open your browser at http://localhost:5001 to confirm your Flask app is live.
This built-in Flask server is for development only. For production deployments, use a WSGI server like Gunicorn or uWSGI . 6. Command Reference Command Description docker build -t flask-docker-demo .Build the Docker image for the Flask app docker imagesList all local Docker images docker run -p HOST:5000 flask-...Run container, mapping host port to container docker run -p 5001:5000 flask-docker-demoRemap host port if default is unavailable
Next Steps
Commit your changes and push to GitHub.
Integrate with a CI/CD pipeline for automated builds.
Deploy to a cloud platform knowing your container behaves the same as it does locally.
Links and References 
