Pulling from private registries in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Docker login command?

You run the command docker login myprivateregistry.com -u user123 -p wrongpassword. What will be the output?

Docker

docker login myprivateregistry.com -u user123 -p wrongpassword

A Error response from daemon: Get "https://myprivateregistry.com/v2/": unauthorized: incorrect username or password

B Syntax error: unexpected argument '-p'

C Login Succeeded

D Warning: Using --password via the CLI is insecure. Please use --password-stdin.

💡 Hint

Think about what happens if you provide a wrong password to a private registry login.

If the password is incorrect, Docker will reject the login and show an unauthorized error message.

❓ Configuration

intermediate

Which Docker configuration file stores credentials for private registries?

You want Docker to remember your login credentials for a private registry. Which file does Docker use to store these credentials by default?

A ~/.docker/credentials.json

B /etc/docker/daemon.json

C ~/.docker/config.json

D /var/lib/docker/registry.json

💡 Hint

It's a JSON file in your home directory under .docker folder.

Docker stores authentication tokens and credentials in the config.json file inside the .docker directory in the user's home folder.

❓ Troubleshoot

advanced

Why does 'docker pull' fail with 'unauthorized: authentication required' even after login?

You logged in successfully to a private registry using docker login. But when you run docker pull myprivateregistry.com/myimage:latest, it fails with unauthorized: authentication required. What is the most likely cause?

A Your Docker client version is outdated

B Docker daemon is not running

C You forgot to run 'docker logout' before pulling

D The image name or tag is incorrect or does not exist in the registry

💡 Hint

Think about what happens if you try to pull an image that is not available or misspelled.

If the image or tag does not exist or is misspelled, the registry denies access with an authentication error even if login succeeded.

🔀 Workflow

advanced

What is the correct sequence to pull a private image securely?

Arrange the steps in the correct order to pull an image from a private Docker registry securely.

A 2,1,3,4

B 1,2,3,4

C 1,3,2,4

D 3,1,2,4

💡 Hint

You must login before pulling private images.

First login authenticates you, then pull downloads the image, verify it exists locally, then run it.

✅ Best Practice

expert

Which method is the most secure for automating Docker login in CI pipelines?

You want to automate Docker login to a private registry in a CI pipeline without exposing your password in logs or code. Which method is best?

A Use 'echo $PASSWORD | docker login --username user --password-stdin' to avoid password in command line

B Commit your password in a config.json file inside the repository

C Store credentials in environment variables and pass them with 'docker login' command

D Use 'docker login -u user -p password' directly in the pipeline script

💡 Hint

Avoid exposing passwords in command arguments or code files.

Using --password-stdin reads the password from standard input, preventing it from appearing in process lists or logs.

Practice

1. What is the first step to pull an image from a private Docker registry?

easy

A. Run docker login with the registry URL to authenticate.

B. Directly run docker pull without authentication.

C. Create a Dockerfile to build the image locally.

D. Restart the Docker daemon.

Solution

1. Step 1: Understand private registry access

   Private registries require authentication before pulling images to ensure security.

2. Step 2: Use the correct command for authentication

   The docker login command with the registry URL authenticates the user.

3. Final Answer:

   Run docker login with the registry URL to authenticate. -> Option A

4. Quick Check:

   Authentication = docker login [OK]

Hint: Always login first to private registries before pulling images [OK]

Common Mistakes:

• Trying to pull without login causes permission errors
• Confusing image build with pulling
• Restarting Docker daemon is unrelated

2. Which of the following is the correct syntax to login to a private Docker registry at myregistry.example.com?

easy

A. docker login myregistry.example.com

B. docker pull myregistry.example.com

C. docker push myregistry.example.com

D. docker build myregistry.example.com

Solution

1. Step 1: Identify the login command

   The docker login command is used to authenticate to a registry.

2. Step 2: Use the registry URL as argument

   Adding the registry URL after docker login targets that specific private registry.

3. Final Answer:

   docker login myregistry.example.com -> Option A

4. Quick Check:

   Login syntax = docker login [OK]

Hint: Use 'docker login' plus registry URL to authenticate [OK]

Common Mistakes:

• Using pull or push commands instead of login
• Omitting the registry URL when needed
• Confusing build command with login

3. What will be the output if you run the following commands in order?

docker login myregistry.example.com
docker pull myregistry.example.com/myapp:latest

medium

A. Error: unauthorized: authentication required

B. Image 'myapp:latest' downloaded successfully from myregistry.example.com

C. Error: repository not found

D. Docker daemon restart required

Solution

1. Step 1: Authenticate with the private registry

   The docker login command authenticates the user successfully.

2. Step 2: Pull the image after authentication

   Since the user is logged in, docker pull downloads the private image without errors.

3. Final Answer:

   Image 'myapp:latest' downloaded successfully from myregistry.example.com -> Option B

4. Quick Check:

   Login then pull = success [OK]

Hint: Login first, then pull to avoid unauthorized errors [OK]

Common Mistakes:

• Pulling without login causes unauthorized errors
• Assuming image is public without checking
• Expecting daemon restart after login

4. You tried to pull an image from a private registry but got this error:
Error response from daemon: unauthorized: authentication required
What is the most likely cause and fix?

medium

A. The image name is misspelled; correct the image name.

B. Your internet connection is down; check network.

C. Docker daemon is not running; restart Docker service.

D. You forgot to run docker login before pulling; run login first.

Solution

1. Step 1: Analyze the error message

   The error says authentication is required, meaning no valid login was done.

2. Step 2: Apply the fix by authenticating

   Running docker login with the registry URL will authenticate and fix the error.

3. Final Answer:

   You forgot to run docker login before pulling; run login first. -> Option D

4. Quick Check:

   Unauthorized error = missing login [OK]

Hint: Unauthorized error means login missing; run docker login [OK]

Common Mistakes:

• Assuming image name typo causes unauthorized error
• Restarting Docker daemon unnecessarily
• Blaming network without checking login

5. You want to automate pulling a private image from registry.example.com in a script. Which sequence of commands correctly handles authentication and pulling?

hard

A. docker build registry.example.com/myimage:latest && docker login registry.example.com

B. docker pull registry.example.com/myimage:latest && docker login registry.example.com

C. docker login registry.example.com && docker pull registry.example.com/myimage:latest

D. docker logout registry.example.com && docker pull registry.example.com/myimage:latest

Solution

1. Step 1: Authenticate before pulling

   Use docker login first to authenticate to the private registry.

2. Step 2: Pull the image after successful login

   Run docker pull to download the private image after authentication.

3. Final Answer:

   docker login registry.example.com && docker pull registry.example.com/myimage:latest -> Option C

4. Quick Check:

   Login then pull = correct sequence [OK]

Hint: Login first, then pull in scripts using && [OK]

Common Mistakes:

• Pulling before login causes failure
• Using build instead of pull for images
• Logging out before pulling removes credentials