Why Building images in CI pipeline in Docker? - Purpose & Use Cases

The Big Idea

What if your software images built themselves perfectly every time you change code?

The Scenario

Imagine you have to manually build a software image every time a developer pushes code. You open your terminal, type commands, wait, and then upload the image yourself. This happens for every small change.

The Problem

This manual way is slow and tiring. You might forget a step or make a typo. It's easy to miss updates or build wrong versions. This causes delays and bugs in the software everyone uses.

The Solution

Building images in a CI pipeline automates this process. Every time code changes, the system builds the image automatically, checks it, and prepares it for use. This saves time and avoids human mistakes.

Before vs After

✗ Before

docker build -t myapp:latest .
docker push myapp:latest

✓ After

ci_pipeline:
  steps:
    - build_image:
        image: myapp:latest
        context: .
    - push_image:
        image: myapp:latest

What It Enables

It enables fast, reliable, and consistent software delivery without manual effort.

Real Life Example

When a developer fixes a bug, the CI pipeline builds a new image automatically and deploys it, so users get the fix quickly without waiting.

Key Takeaways

Manual image building is slow and error-prone.

CI pipelines automate image building and pushing.

This leads to faster and safer software updates.

Practice

1. What is the main purpose of building Docker images in a CI pipeline?

easy

A. To automate packaging the app for consistent deployment

B. To manually upload files to the server

C. To write application code faster

D. To monitor server health

Solution

1. Step 1: Understand CI pipeline role

   CI pipelines automate tasks like testing and packaging to ensure consistency.

2. Step 2: Identify Docker image purpose

   Docker images package the app and its environment for reliable deployment.

3. Final Answer:

   To automate packaging the app for consistent deployment -> Option A

4. Quick Check:

   CI builds images to package apps consistently [OK]

Hint: CI builds images to package apps consistently [OK]

Common Mistakes:

• Confusing image building with code writing
• Thinking CI uploads files manually
• Mixing deployment with monitoring

2. Which of the following is the correct command to build a Docker image named myapp with tag v1 from the current directory?

easy

A. docker build -t myapp:v1 .

B. docker create -t myapp:v1 .

C. docker run -t myapp:v1 .

D. docker push -t myapp:v1 .

Solution

1. Step 1: Identify Docker build syntax

   The correct syntax to build an image is docker build -t name:tag path.

2. Step 2: Match command to syntax

   docker build -t myapp:v1 . uses docker build -t myapp:v1 . which matches the syntax and current directory.

3. Final Answer:

   docker build -t myapp:v1 . -> Option A

4. Quick Check:

   Build command uses 'docker build -t name:tag path' [OK]

Hint: Use 'docker build -t name:tag path' to build images [OK]

Common Mistakes:

• Using 'docker create' instead of 'docker build'
• Confusing 'docker run' with building images
• Trying to push before building

3. Given this CI script snippet:

docker build -t myapp:latest .
docker tag myapp:latest myapp:build-123

What will be the result after running these commands?

medium

A. An error because tagging must happen before building

B. Only one image named 'myapp:build-123' exists

C. Two images: 'myapp:latest' and 'myapp:build-123' pointing to the same image

D. Two images with different contents

Solution

1. Step 1: Understand docker build output

   The build creates an image tagged 'myapp:latest'.

2. Step 2: Understand docker tag effect

   Tagging 'myapp:latest' as 'myapp:build-123' creates a new tag pointing to the same image.

3. Final Answer:

   Two images: 'myapp:latest' and 'myapp:build-123' pointing to the same image -> Option C

4. Quick Check:

   Tag duplicates image reference, no new image created [OK]

Hint: Tagging duplicates image reference, no new image built [OK]

Common Mistakes:

• Thinking tagging creates a new image
• Assuming error if tag after build
• Believing images have different contents after tag

4. A CI pipeline script has this line:

docker build -t myapp .

But the build fails with error: "Cannot locate Dockerfile". What is the likely cause?

medium

A. The tag name 'myapp' is invalid

B. Dockerfile is missing in the current directory

C. The docker daemon is not running

D. The build command syntax is wrong

Solution

1. Step 1: Analyze error message

   "Cannot locate Dockerfile" means Docker can't find the Dockerfile in the build context.

2. Step 2: Check build context and files

   Since the path is '.', Docker expects a Dockerfile in the current directory, which is missing.

3. Final Answer:

   Dockerfile is missing in the current directory -> Option B

4. Quick Check:

   Missing Dockerfile causes this error [OK]

Hint: Check Dockerfile presence in build directory [OK]

Common Mistakes:

• Blaming tag name for Dockerfile error
• Assuming daemon not running without checking
• Thinking build syntax is incorrect

5. In a CI pipeline, you want to tag Docker images with the build number stored in the environment variable BUILD_ID. Which command correctly builds and tags the image myapp with this dynamic tag?

hard

A. docker build -t 'myapp:${BUILD_ID}' .

B. docker build -t myapp:BUILD_ID .

C. docker build -t myapp:$(BUILD_ID) .

D. docker build -t myapp:$BUILD_ID .

Solution

1. Step 1: Understand shell variable syntax

   In shell scripts, environment variables are accessed with $VAR or ${VAR}.

2. Step 2: Identify correct variable usage in command

   docker build -t myapp:$BUILD_ID . uses $BUILD_ID which is valid and common in CI scripts.

3. Final Answer:

   docker build -t myapp:$BUILD_ID . -> Option D

4. Quick Check:

   Use $VAR to insert env variable in commands [OK]

Hint: Use $VAR to insert env variable in commands [OK]

Common Mistakes:

• Using literal 'BUILD_ID' instead of variable
• Using incorrect syntax like $(BUILD_ID)
• Forgetting to expand variable in tag