How to Create a CI/CD Pipeline for CircleCI Docker Images

Published:16 March 2022 - 7 min. read

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Muhammed Ali

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If you’re looking for a way to test and deploy your CircleCI Docker images, creating a CI/CD pipeline can take you a long way to improve software delivery.

In this tutorial, you’ll learn how you can potentially develop a CI/CD pipeline for your Docker image using CircleCI.

Read on and deploy your Docker images, only if they pass the test!


This tutorial will be a hands-on demonstration. If you’d like to follow along, be sure you have the following:

Creating a Docker Image of a Python-Based Application

Before jumping to creating a CI/CD pipeline, you’ll first build a Docker image based on a Python application that displays texts on a web browser. You’ll briefly go through how the application is built, write the test cases for the application and then containerize the application.

1. Open your terminal, and run the following command to install Flask and Pytest. Flask is a framework used to build web applications in Python while Pytest is used to write tests Python code.

pip install flask pytest

2. Next, create a project directory with your preferred name, but the project directory is called flask-circleci in this demo. This directory is where you’ll store your resources for this tutorial.

mkdir flask-circleci

3. Create a file called flask-circleci/ and populate the following code.

The code below displays the “Welcome to my Flask App” text on a home page in a browser.

from flask import Flask

app = Flask(__name__) #create an instance of the Flask class you just imported

def main():
    # display a string in the home page
    return "Welcome to my Flask App"

4. Now, create another file called flask-circleci/ and copy/paste the following code.

The code below creates a test client for your application.

Pytest requires that the name of your test file begins with test_ as demonstrated below.

from main import app

def test_main():
		# Creates a test client for this application.
    response = app.test_client().get('/') 

		# assert the stutus code of the page('/') is 200
    assert response.status_code == 200 
		# assert the return statement to the page
    assert == b'Welcome to my Flask App' 

5. Run the following commands to navigate to your project directory (flask-circleci), run the test for your project.

cd flask-circleci

If the test succeeds, you’ll get 100% progress, as shown below.

Running Test for the Project
Running Test for the Project

6. Next, create a text file (requirements.txt) to document your dependencies in the root directory and populate the text below. This file is essential when building your Docker image so that it works properly.


7. Lastly, create a new file (flask-circleci/Dockerfile), and write the instructions below to the Dockerfile to build your Docker image.

The instructions in the Dockerfile perform the following:

  • Create a file in your image.
  • Copies all the files in the files in the current directory (flask-circleci).
  • Run your code
# set the Base Image from which your image will be built on
FROM python:3.8 
# create a directory called flask-circleci in root. 
# This directory will contain the code which currently resides in
RUN mkdir /flask-circleci

# make /flask-circleci the working directory
WORKDIR /flask-circleci

# copy your requirements file to the directory you just created
COPY requirements.txt /flask-circleci 

RUN pip install -r requirements.txt

# copy the current directory in you local machine to /flask-circleci in your image
ADD . /flask-circleci


CMD python

Creating a CircleCI CI/CD Pipeline

Now you have your Docker image ready, you’ll create a CI/CD pipeline to test your code, and if all tests pass the Docker image and pushed to Docker Hub. Docker Hub is a service like GitHub, but for Docker images, that helps you find and share container images with your team or other developers.

To create the CI/CD pipeline, follow the steps below:

1. Open your favorite web browser and log in to your Docker Hub account.

2. On the Docker Hub home page, click on the Create Repository option to create a repository on Docker Hub.

Creating a Repository
Creating a Repository

3. Set a unique name for your repository. This tutorial uses a repository called circleci-tutorial, as shown below.

Naming and Creating the Docker Hub Repository
Naming and Creating the Docker Hub Repository

4. Create a new file (flask-circleci/config.yml) at the root of your project, and copy/paste the code below to the config.yml file.

The code below gets a ready-made Python image and uses it to install the dependencies and run the unit tests to build and push your Docker image to your repository Docker Hub.

Typically, for a live project, you’d want to put your username and password protected within CircleCI or maybe in a .env file.

version: 2  # Version of CircleCI
      - image: python:3.8
      - checkout

      - run:
        # creates a virtual environment for you project, 
				# install dependencies in it and run tests
          name: Run unit tests
          command: |
            python3 -m venv venv
            . venv/bin/activate
            pip install -r requirements.txt
			# Creates a remote Docker environment configured to execute Docker commands.
      - setup_remote_docker 
      - run:
        # installs a docker client that will be used to run the docker commands
          name: Install Docker client
          command: |
            set -x
            curl -L -o /tmp/docker-$VER.tgz$VER.tgz
            tar -xz -C /tmp -f /tmp/docker-$VER.tgz
            mv /tmp/docker/* /usr/bin
      - run:
         # Builds a docker image to push to Docker Hub
				 # Tag (-t) pattern is below, where CircleCI 
				 # to get the Dockerfile from the current directory.
         # <docker_hub_username/docker-hub-password:version>.
          name: Build Docker image
          command: docker build -t khabdrick/circleci-tutorial:v1 .

      - run:
        # Pushes the Docker image you created to the Docker Hub.
        # Replace khabdrick/circleci-tutorial:v1 with the 
				# <docker_hub_username/docker-hub-password:version> you used to build the image above
          name: Push to Docker Hub
          command: |
            docker login -u unsername -p password
            docker push khabdrick/circleci-tutorial:v1

5. Finally, run the following commands on your project’s root to commit and push the code (flask-circleci/config.yml) to your GitHub repository.

git add . # adds changes to staging area
git commit -m "update" # commits your changes
git push # Push to GitHub

Activating the CircleCI CI/CD Pipeline

You’ve just created your CircleCI CI/CD Pipeline, but right now, it’s just sitting there and not doing much. You’ll have to activate your pipeline by setting up a project in your CircleCI account.

1. Log in to your CircleCI account with your GitHub account on your web browser.

2. Next, click the Projects tab on the left panel, and click the Set Up Project button on the right side of your project to activate the CircleCI pipeline on the project.

Since you logged in to CircleCI with your GitHub account, your projects sync with CircleCI, like in the image below.

Setting up a Project
Setting up a Project

3. Navigate back to the Dashboard tab and you’ll see the Success status. The Success status indicates everything ran as expected, and the image has been pushed to your Docker Hub repository.

Click on build to see all the steps that ran in the pipeline.

Viewing Project Status
Viewing Project Status

Below, you can see the list of steps CircleCI took to run the pipeline.

Viewing Steps that Ran the Pipeline
Viewing Steps that Ran the Pipeline

4. Finally, switch back to your Docker Hub repository, and you’ll see the image you pushed to Docker Hub through the CircleCI pipeline.

Viewing Pushed Image in Docker Hub Repository
Viewing Pushed circleci docker images in Docker Hub Repository

Pushing Changes to GitHub to Trigger the CircleCI Pipeline

By now, you already have a working CircleCI pipeline. But perhaps you made changes to your project. If so, how would you know the pipeline still works? You can trigger CircleCI by pushing changes to your GitHub repository and see if the test will succeed.

1. On your local machine, replace the code you have in with the code below. The code below purposely ensures the test fails because the status code changed from 200 to 400.

from main import app

def test_main():
		# Creates a test client for this application.
    response = app.test_client().get('/') 

		# assert the status code of the page('/') is 400 when it isn't, 
		# causing the test to fail.
    assert response.status_code == 400 
		# assert the return statement ton the page
    assert == b'Flask App' 

2. Next, replace the steps in the config.yml that builds the Docker image and pushes it to Docker Hub with the code below.

In the code below, you’re just changing the version to ensure sure only new changes are pushed to your GitHub repository.

      - run:
          name: Build Docker image
          command: docker build -t khabdrick/circleci-tutorial:v2 .

      - run:
          name: Push to DockerHub
          command: |
            docker login -u khabdrick -p Muhammed-1998
            docker push khabdrick/circleci-tutorial:v2

3. Run the following commands on your project’s root to commit and push the code to your GitHub repository.

git add . # adds changes to staging area
git commit -m "v2" # commits your changes
git push # Push to GitHub

4. Now, navigate to your CircleCI dashboard, and you’ll see that the build Failed, as shown below.

Click on build to see the steps taken to push the changes and confirm what caused the build to fail.

Viewing Project Status (Failed)
Viewing Project Status (Failed)
Showing Failed Tests
Showing Failed Tests

5. Lastly, navigate to your Docker Hub repository and confirm the changes were not pushed.

As you can see in the image below, v2 wasn’t uploaded because the test failed.

Viewing Repository in Docker Hub
Viewing Repository in Docker Hub


Throughout this tutorial, you’ve learned how to write tests, and Dockerize your Python application. You’ve created a CI/CD pipeline with CircleCI that runs the test for your application and pushes your Docker image to Docker Hub.

At this point, you already have a foundational knowledge of developing a CI/CD pipeline with CircleCI. Now, how do you intend to use this newfound knowledge? Perhaps creating a CI/CD pipeline that deploys your Docker image to Heroku?

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