Building a Simple Jenkins CI/CD Pipeline for a Dockerized Flask App
I recently built my first real-world CI/CD pipeline using Jenkins, Docker, and Flask — and I want to share the journey with you.
What I Wanted to Achieve
Automatically build, test, and deploy a Flask app every time I push code
- Use Jenkins for automation
- Containerize the app with Docker
- Run everything on my local machine
How I Did It
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Set up the project Created a basic Flask app and wrote tests using pytest. I also added a requirements.txt file listing dependencies.
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Created a Jenkinsfile pipeline Defined stages:
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Clone the GitHub repo
- Install dependencies (pip install -r requirements.txt)
- Run tests (pytest)
- Build Docker image
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Run Docker container
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Installed Jenkins and Docker on my RHEL machine Made sure Jenkins can run Docker commands (added Jenkins user to Docker group).
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Pushed the code to GitHub Hosted the repo publicly to make integration easier.
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Connected Jenkins with GitHub using Webhooks and Ngrok Since Jenkins runs locally, I used ngrok to expose my localhost to the internet. This allowed GitHub to notify Jenkins instantly when I push changes.
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Configured Jenkins job to trigger on webhook events Enabled GitHub hook trigger for GITScm polling in Jenkins.
Challenges I Faced
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Git clone error — Jenkins couldn't find the repo initially because I pointed it to a local folder instead of the GitHub URL.
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pytest not found — Jenkins didn't have pytest installed by default. Fixed by adding pytest to requirements.txt and installing dependencies in the pipeline.
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Python import errors — Tests couldn’t find my Flask app modules because PYTHONPATH wasn’t set. Added export PYTHONPATH=$PWD before running tests.
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Werkzeug version incompatibility — Tests failed with AttributeError: module 'werkzeug' has no attribute 'version'. Fixed by pinning Werkzeug to a version below 3.0 in requirements.txt.
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Local Jenkins not reachable from GitHub — Solved by using ngrok to create a public tunnel to my localhost.
The Outcome
Now, every time I push code to GitHub:
- Jenkins automatically clones the repo
- Installs dependencies
- Runs tests
- Builds a Docker image
- Runs the Flask app in Docker
- All without me clicking a button. It’s a fully automated pipeline on my local machine!
Why This Matters
This project taught me how real CI/CD pipelines work in practice, and gave me a solid foundation to build more advanced DevOps skills. If you're starting your DevOps journey, I recommend building a similar pipeline. It covers all core concepts and tools without being overwhelming.