Pushing images from CI in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Docker login command in CI?

You run this command in your CI pipeline to authenticate with Docker Hub:

docker login -u $DOCKER_USER -p $DOCKER_PASS

What will be the output if the credentials are correct?

Docker

docker login -u $DOCKER_USER -p $DOCKER_PASS

A Error: Cannot perform an interactive login from a non TTY device

B Error: Invalid username or password

C Warning: Using --password via the CLI is insecure

D Login Succeeded

💡 Hint

Think about what Docker shows when login is successful.

When Docker login succeeds, it prints 'Login Succeeded'. Other messages indicate errors or warnings.

❓ Configuration

intermediate

Which GitHub Actions step correctly pushes a Docker image to Docker Hub?

You want to push a Docker image from GitHub Actions to Docker Hub. Which step configuration will work correctly?

A

- name: Push image
  run: |
    docker push myuser/myimage:latest
  env:
    DOCKER_USER: ${{ secrets.DOCKER_USER }}
    DOCKER_PASS: ${{ secrets.DOCKER_PASS }}

B

- name: Push image
  run: |
    docker push myuser/myimage
  env:
    DOCKER_USER: ${{ secrets.DOCKER_USER }}
    DOCKER_PASS: ${{ secrets.DOCKER_PASS }}

C

- name: Push image
  run: |
    docker push myuser/myimage:latest

D

- name: Push image
  run: |
    docker push myuser/myimage:latest
  env:
    USERNAME: ${{ secrets.DOCKER_USER }}
    PASSWORD: ${{ secrets.DOCKER_PASS }}

💡 Hint

Check if the image tag is specified and environment variables match Docker login.

Option A correctly pushes the tagged image and sets environment variables matching Docker Hub credentials.

❓ Troubleshoot

advanced

Why does this CI pipeline fail to push the Docker image?

In your CI pipeline, you run:

docker build -t myuser/myimage:latest .
docker push myuser/myimage:latest

The push fails with:
denied: requested access to the resource is denied
What is the most likely cause?

A The Dockerfile has syntax errors causing build failure

B Docker Hub credentials are missing or incorrect in the CI environment

C The image tag 'latest' is not allowed by Docker Hub

D The Docker daemon is not running on the CI server

💡 Hint

Think about permissions needed to push images to Docker Hub.

The error means the CI cannot authenticate to Docker Hub, so credentials are missing or wrong.

🔀 Workflow

advanced

What is the correct order of steps to push a Docker image in a CI pipeline?

Arrange these steps in the correct order to push a Docker image from a CI pipeline:

A 4,3,1,2

B 4,3,2,1

C 4,2,3,1

D 2,4,3,1

💡 Hint

Think about setting credentials before login, then build, then push.

First set credentials, then login, then build the image, then push it.

✅ Best Practice

expert

Which practice is best to securely push Docker images from CI?

You want to push Docker images from your CI pipeline securely. Which practice is best?

A Store Docker Hub credentials as encrypted secrets in the CI system and use docker login with them

B Hardcode Docker Hub username and password in the CI pipeline script for simplicity

C Push images without authentication to a public Docker Hub repository

D Use plain text environment variables in the CI logs for easy debugging

💡 Hint

Think about security and secret management best practices.

Storing credentials securely as encrypted secrets and using them in docker login is best practice.

Practice

1. What is the main purpose of pushing Docker images from a CI pipeline?

easy

A. To delete old images from the local Docker cache

B. To run containers automatically on the local machine

C. To upload built images to a remote registry for sharing and deployment

D. To build images only on the developer's computer

Solution

1. Step 1: Understand CI pipeline role

   A CI pipeline automates building and testing code changes, including Docker images.

2. Step 2: Purpose of pushing images

   Pushing images uploads them to a remote registry so others can use or deploy them.

3. Final Answer:

   To upload built images to a remote registry for sharing and deployment -> Option C

4. Quick Check:

   Pushing images = Upload to registry [OK]

Hint: Pushing means sending images to a remote place [OK]

Common Mistakes:

• Confusing pushing with running containers
• Thinking pushing deletes images
• Assuming pushing only builds images locally

2. Which command correctly pushes a Docker image named myapp:latest to Docker Hub in a CI script?

easy

A. docker publish myapp:latest

B. docker upload myapp:latest

C. docker send myapp:latest

D. docker push myapp:latest

Solution

1. Step 1: Identify Docker push syntax

   The correct command to upload images to a registry is docker push <image>.

2. Step 2: Verify other options

   Commands like upload, send, and publish are not valid Docker commands.

3. Final Answer:

   docker push myapp:latest -> Option D

4. Quick Check:

   Push = docker push [OK]

Hint: Use 'docker push' to send images to registry [OK]

Common Mistakes:

• Using 'docker upload' instead of 'docker push'
• Confusing push with publish or send commands
• Missing image tag in push command

3. Given this CI script snippet:

docker build -t myuser/myapp:1.0 .
docker login -u myuser -p $DOCKER_PASS
docker push myuser/myapp:1.0

What will happen if $DOCKER_PASS is empty?

medium

A. The push will fail due to authentication error

B. The image will push successfully without login

C. The build command will fail before push

D. The image will be pushed anonymously

Solution

1. Step 1: Understand docker login with empty password

   If $DOCKER_PASS is empty, login will fail because no valid password is provided.

2. Step 2: Effect on docker push

   Without successful login, pushing to Docker Hub requires authentication and will fail.

3. Final Answer:

   The push will fail due to authentication error -> Option A

4. Quick Check:

   Empty password = login fail = push fail [OK]

Hint: Empty password means login fails, so push fails [OK]

Common Mistakes:

• Assuming push works without login
• Thinking build fails due to empty password
• Believing anonymous push is allowed

4. You have this CI step:

docker push myuser/myapp:latest

But the push fails with denied: requested access to the resource is denied. What is the most likely cause?

medium

A. The image tag latest is invalid

B. You forgot to run docker login before pushing

C. The Dockerfile has syntax errors

D. The Docker daemon is not running

Solution

1. Step 1: Analyze error message meaning

   The error means Docker Hub denied access, usually due to missing or wrong authentication.

2. Step 2: Check common causes

   Not running docker login before push is the most common cause of access denial.

3. Final Answer:

   You forgot to run docker login before pushing -> Option B

4. Quick Check:

   Access denied = missing login [OK]

Hint: Always login before pushing images [OK]

Common Mistakes:

• Blaming image tag for access denied
• Assuming Dockerfile errors cause push failure
• Ignoring Docker daemon status

5. In a CI pipeline, you want to push a Docker image only if the build and tests succeed. Which is the best way to ensure this?

hard

A. Add a separate job that depends on build and test jobs and pushes the image

B. Push the image immediately after the build step without waiting for tests

C. Push the image before running tests to save time

D. Push the image manually after the pipeline finishes

Solution

1. Step 1: Understand CI job dependencies

   To push only after build and tests succeed, create a job that runs after those jobs complete successfully.

2. Step 2: Why other options fail

   Pushing before tests risks pushing broken images; manual push loses automation benefits.

3. Final Answer:

   Add a separate job that depends on build and test jobs and pushes the image -> Option A

4. Quick Check:

   Push after success = dependent job [OK]

Hint: Use dependent jobs to control push timing [OK]

Common Mistakes:

• Pushing before tests complete
• Manual push breaks automation
• Ignoring job dependencies in CI