🚀 Dockerized Flask App Deployment via Jenkins CI/CD on Localhost

Automate Docker image creation and deployment for a Python Flask app using Jenkins pipelines, built from scratch on WSL2.

🧠 Introduction

As part of my DevOps learning journey, I decided to take on a real-world automation challenge:

📌 Build a Jenkins pipeline from scratch to deploy a Dockerized Flask app on localhost!

“Automate Docker Image Creation and Deployment using Jenkins Pipeline — no pre-built images allowed.”

🔧 What I Built

✅ Project Overview

• Build a Flask app Docker image

• Automate the entire process using Jenkins pipeline

• Expose the container at localhost:5000

• Push image to self-hosted registry (optional)

🧰 Tech Stack

📁 File Structure

📦 flask-docker-jenkins/
 ┣ 📂 app/
 ┃ ┗ 📄 app.py
 ┣ 📄 Dockerfile
 ┣ 📄 Jenkinsfile

💡 The Flask App (app.py)

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return "✅ Hello from Flask App via Docker + Jenkins!"

if __name__ == '__main__':
    app.run(host="0.0.0.0", port=5000)

🐳 Dockerfile

FROM python:3.9
WORKDIR /app
COPY ./app /app
RUN pip install -r requirements.txt
CMD ["python3", "app.py"]

⚙️ Jenkinsfile (CI/CD Pipeline) - get it from github -https://github.com/HARSHITHA-G-M/Automate-Docker-Image-Creation-and-Deployment-using-Jenkins-Pipeline.git

😵‍💫 Errors I Faced (and Fixed)

❌ Jenkins service failed to start

✅ Fixed by installing correct Java version:

sudo apt install openjdk-17-jdk -y

❌ App not showing at localhost:5000

✅ Realized Jenkins was only building the image, not running it. Added docker run in pipeline.

❌ Port conflict:

Bind for 0.0.0.0:5000 failed: port is already allocated

✅ Found out registry container was using it. Stopped it:

docker stop registry && docker rm registry

✅ Final Output

Once fixed, the Jenkins pipeline:

• Builds the Docker image

• Cleans up old containers

• Runs the Flask app

• App accessible at:
  🔗 http://localhost:5000

📌 Key Takeaways

✅ Docker alone isn't enough — CI/CD automation is where DevOps shines
✅ Always check running containers with docker ps
✅ Port conflicts are real and sneaky — clean up before re-running
✅ Jenkins won't run your app unless you explicitly add that step!
✅ Working from scratch gives you production-like debugging experience

🧠 Lesson You Learned (and Should Highlight in Blog/LinkedIn)

“Even if CI/CD automation works at script level, we must validate app health post-deployment — especially in local setups where ports and services can clash.”