Building images in CI pipeline in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

To automate the creation of consistent, tested application environments that can be deployed reliably.

beginner

Which file usually contains the instructions to build a Docker image?

The Dockerfile contains step-by-step instructions to build a Docker image.

beginner

Name a common CI tool that can be used to build Docker images automatically.

Jenkins, GitHub Actions, GitLab CI, and CircleCI are popular CI tools that support Docker image builds.

intermediate

Why is it important to tag Docker images in a CI pipeline?

Tagging helps identify image versions clearly, making it easier to deploy specific builds and track changes.

beginner

What command is used in a CI script to build a Docker image from a Dockerfile?

The command is: docker build -t image_name:tag .

What does the command 'docker build -t myapp:latest .' do in a CI pipeline?

A Pushes the Docker image 'myapp:latest' to a registry

B Runs a Docker container named 'myapp' with the tag 'latest'

C Builds a Docker image named 'myapp' with the tag 'latest' from the current directory

D Deletes the Docker image named 'myapp:latest'

The 'docker build' command creates an image from the Dockerfile in the current directory, tagging it as 'myapp:latest'.

Why should Docker images be built in a CI pipeline rather than manually?

A Because manual builds are faster

B To ensure consistent, repeatable builds and automate testing

C To avoid using Dockerfiles

D To skip testing steps

CI pipelines automate builds to make sure they are consistent and tested every time code changes.

Which file must be present in the project root for Docker to build an image?

A Dockerfile

B docker-compose.yml

C README.md

D build.sh

Dockerfile contains the instructions needed to build the Docker image.

In a CI pipeline, what is a common next step after building a Docker image?

A Push the image to a Docker registry

B Delete the image immediately

C Run the image without testing

D Ignore the image

Pushing the image to a registry makes it available for deployment or sharing.

What is the benefit of using tags like 'latest' or version numbers on Docker images in CI?

A They prevent image building

B They make images run faster

C They delete old images automatically

D They help track and deploy specific image versions

Tags identify image versions clearly, aiding deployment and rollback.

Explain the steps to build and push a Docker image in a CI pipeline.

Think about how automation helps create and share images.

Why is automating Docker image builds in CI important for software delivery?

Consider benefits of automation and reliability.

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