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A Your Docker client version is outdated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.