Building images in CI pipeline in Docker - Commands & Configuration

Introduction

Building images in a CI pipeline automates creating container images whenever code changes. This ensures your app is always packaged and ready to run in any environment without manual steps.

When you want to automatically create a new app version image after every code update.

When you need to test your app inside a container before deploying it.

When you want to push the built image to a registry for sharing or deployment.

When you want to catch build errors early by building images in a controlled environment.

When you want to keep your deployment process consistent and repeatable.

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
CMD ["python", "app.py"]

This Dockerfile starts from a small Python 3.11 image.

It sets the working folder to /app inside the container.

It copies the requirements file and installs Python packages.

Then it copies the app code and sets the command to run the app.

Commands

This command builds a Docker image named 'my-app-image' with tag '1.0' from the current folder where the Dockerfile is located.

Terminal

docker build -t my-app-image:1.0 .

Expected Output

Sending build context to Docker daemon  12.3MB

Step 1/5 : FROM python:3.11-slim

 ---> 123abc456def

Step 2/5 : WORKDIR /app

 ---> Using cache

 ---> 789def012abc

Step 3/5 : COPY requirements.txt .

 ---> Using cache

 ---> 345abc678def

Step 4/5 : RUN pip install --no-cache-dir -r requirements.txt

 ---> Running in 456def789abc

Collecting flask

Installing collected packages: flask

Successfully installed flask-2.2.2

Removing intermediate container 456def789abc

 ---> 901abc234def

Step 5/5 : COPY . .

 ---> 567def890abc

Successfully built 567def890abc

Successfully tagged my-app-image:1.0

→

-t - Assigns a name and tag to the image for easy reference.

This command lists the Docker images filtered by the name 'my-app-image' to verify the image was built successfully.

Terminal

docker images my-app-image

Expected Output

REPOSITORY      TAG       IMAGE ID       CREATED          SIZE

my-app-image    1.0       567def890abc   10 seconds ago   120MB

This runs the built image in a container to test if the app starts correctly. The --rm flag removes the container after it stops.

Terminal

docker run --rm my-app-image:1.0

Expected Output

 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

→

--rm - Automatically removes the container after it exits to keep the system clean.

Key Concept

If you remember nothing else from this pattern, remember: building images in CI automates packaging your app so it runs the same everywhere.

Common Mistakes

Not specifying the build context as '.' in the docker build command.

Docker won't find the Dockerfile or app files, causing the build to fail.

Always include '.' at the end of 'docker build' to specify the current folder as context.

Forgetting to tag the image with -t during build.

Without a tag, the image gets a random ID making it hard to reference later.

Use '-t name:tag' to give your image a clear name and version.

Running the image without testing it first.

You might deploy a broken image if you don't verify it runs correctly.

Run the image locally with 'docker run' to confirm it works before pushing or deploying.

Summary

Use 'docker build -t name:tag .' to create a container image from your app code.

Verify the image exists with 'docker images name'.

Test the image by running it with 'docker run --rm name:tag' to ensure it works.