Docker login and authentication - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of a successful Docker login?

You run the command docker login and enter valid credentials for Docker Hub. What output do you see?

Docker

docker login

A Error: unauthorized: incorrect username or password

B Login Succeeded

C Cannot connect to the Docker daemon

D Warning: login credentials expired

💡 Hint

Think about what message confirms a successful login.

When you login successfully to Docker Hub, the message Login Succeeded is shown.

🧠 Conceptual

intermediate

What does the Docker config file store after login?

After running docker login, Docker stores authentication info locally. What does the config file typically contain?

A Encrypted tokens or credentials for registries

B List of all pulled images

C Docker daemon configuration settings

D Plain text passwords for all registries

💡 Hint

Think about security and what should be stored safely.

Docker stores encrypted tokens or credentials in ~/.docker/config.json after login, not plain passwords.

❓ Troubleshoot

advanced

Why does docker login fail with 'unauthorized' error?

You run docker login with your username and password but get unauthorized: incorrect username or password. What is the most likely cause?

A Docker client version is outdated

B Docker daemon is not running

C Docker Hub service is down

D Incorrect username or password entered

💡 Hint

Check your credentials carefully.

This error means the username or password is wrong. The daemon or client version would cause different errors.

🔀 Workflow

advanced

What is the correct sequence to authenticate and push an image to a private registry?

You want to push a Docker image to a private registry that requires login. Which sequence of commands is correct?

A docker push myimage; docker login registry.example.com; docker tag myimage registry.example.com/myimage

B docker tag myimage registry.example.com/myimage; docker push registry.example.com/myimage; docker login registry.example.com

C docker login registry.example.com; docker tag myimage registry.example.com/myimage; docker push registry.example.com/myimage

D docker login registry.example.com; docker push myimage; docker tag myimage registry.example.com/myimage

💡 Hint

Login must happen before pushing, and tagging must happen before push with the registry prefix.

You must login first, then tag your image with the registry URL, then push it.

✅ Best Practice

expert

Which method is best to securely authenticate Docker in automated CI/CD pipelines?

In a CI/CD pipeline, you want to authenticate Docker to push images without exposing your password. Which method is best practice?

A Use Docker access tokens or secrets stored securely in the pipeline environment

B Use Docker login with username and password interactively during the pipeline

C Store your Docker Hub password in plain text in the pipeline config

D Skip authentication and push images anonymously

💡 Hint

Think about security and automation without manual input.

Using access tokens or secrets securely stored in the pipeline environment is best practice for automated authentication.

Practice

1. What is the main purpose of the docker login command?

easy

A. To authenticate your computer with a Docker registry

B. To start a Docker container

C. To build a Docker image

D. To remove unused Docker images

Solution

1. Step 1: Understand the role of docker login

   The docker login command is used to provide your credentials to a Docker registry so you can access private images.

2. Step 2: Differentiate from other Docker commands

   Starting containers, building images, or removing images are done by other commands like docker run, docker build, and docker image prune.

3. Final Answer:

   To authenticate your computer with a Docker registry -> Option A

4. Quick Check:

   docker login = authenticate registry [OK]

Hint: Login connects you to registry for private image access [OK]

Common Mistakes:

• Confusing login with image build or container start
• Thinking login removes images
• Assuming login runs containers

2. Which of the following is the correct syntax to login to Docker Hub using the command line?

easy

A. docker login username password

B. docker login -u username

C. docker login --user username

D. docker login --user=username password

Solution

1. Step 1: Recall the docker login syntax

   The correct syntax to login is docker login -u username or docker login --username username. The password is prompted securely after running the command.

2. Step 2: Identify the correct option

   docker login -u username uses the short flag -u correctly without exposing the password inline, which is the recommended way.

3. Final Answer:

   docker login -u username -> Option B

4. Quick Check:

   docker login -u username = correct syntax [OK]

Hint: Use -u or --username, password entered after prompt [OK]

Common Mistakes:

• Passing password directly in command
• Using incorrect flags like --user
• Omitting username flag

3. What will be the output of the following command if the login is successful?

docker login -u user123

medium

A. Error: Invalid username or password

B. Cannot connect to the Docker daemon

C. Usage: docker login [OPTIONS]

D. Login Succeeded

Solution

1. Step 1: Understand the expected output of successful login

   When docker login succeeds, it prints Login Succeeded to confirm authentication.

2. Step 2: Differentiate from error messages

   Other options represent errors or usage info, which appear only if login fails or command is incorrect.

3. Final Answer:

   Login Succeeded -> Option D

4. Quick Check:

   Successful login message = Login Succeeded [OK]

Hint: Success message is 'Login Succeeded' exactly [OK]

Common Mistakes:

• Confusing error messages with success
• Expecting no output on success
• Misreading usage info as success

4. You run docker login -u user123 but get the error: Cannot connect to the Docker daemon. What is the most likely cause?

medium

A. Docker daemon is not running on your machine

B. Incorrect username or password

C. You forgot to specify the registry URL

D. Docker CLI version is outdated

Solution

1. Step 1: Analyze the error message

   The error Cannot connect to the Docker daemon means the Docker service (daemon) is not running or accessible.

2. Step 2: Rule out other causes

   Incorrect credentials or missing registry URL cause authentication errors, not daemon connection errors. CLI version issues usually show different messages.

3. Final Answer:

   Docker daemon is not running on your machine -> Option A

4. Quick Check:

   Daemon connection error = daemon not running [OK]

Hint: Daemon errors mean Docker service is off or unreachable [OK]

Common Mistakes:

• Assuming wrong password causes daemon error
• Thinking registry URL is mandatory for Docker Hub login
• Blaming CLI version for connection issues

5. You want to automate Docker login in a CI/CD pipeline without exposing your password in the command line. Which approach is best?

hard

A. Use docker login -u user -p password directly in the script

B. Write your password in a plain text file and read it in the script

C. Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin

D. Skip login and pull images anonymously

Solution

1. Step 1: Understand secure login methods for automation

   Using --password-stdin with environment variables avoids exposing passwords in command history or process lists.

2. Step 2: Evaluate other options for security

   Passing password directly in command or storing in plain text files risks exposure. Skipping login may fail for private images.

3. Final Answer:

   Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin -> Option C

4. Quick Check:

   Use --password-stdin with env vars for secure automation [OK]

Hint: Use --password-stdin and env vars to keep passwords safe [OK]

Common Mistakes:

• Passing password directly in command line
• Storing passwords in plain text files
• Skipping login for private image pulls