Docker engine and runtime - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify Docker Engine Version Output

What is the output of the command docker version --format '{{.Server.Version}}' when Docker Engine version 24.0.2 is installed?

Docker

docker version --format '{{.Server.Version}}'

A 24.0.2

B Client: 24.0.2

C Version: 24.0.2

D Docker Engine 24.0.2

💡 Hint

The --format option outputs only the specified field without extra text.

The command outputs only the server version string exactly as '24.0.2' without any prefix or label.

🧠 Conceptual

intermediate

Docker Runtime Default Behavior

Which statement correctly describes the default runtime used by Docker Engine when running containers?

A Docker uses the Kata Containers runtime by default for enhanced security.

B Docker uses the runc runtime by default to create and run containers.

C Docker uses the cri-o runtime by default for container orchestration.

D Docker uses the containerd runtime by default to run containers.

💡 Hint

Think about the low-level runtime that Docker calls to create containers.

Docker Engine uses runc as the default runtime to create and run containers. containerd is a daemon Docker uses internally, but runc is the runtime that actually runs containers.

❓ Troubleshoot

advanced

Diagnose Docker Engine Startup Failure

A user reports that Docker Engine fails to start with the error message: failed to start daemon: Error initializing network controller: Error creating default "bridge" network: failed to allocate gateway (172.17.0.1): Address already in use. What is the most likely cause?

A Another process is already using the default Docker bridge network IP address range.

B Docker daemon configuration file is missing the bridge network settings.

C Docker Engine version is incompatible with the host kernel version.

D The Docker socket file has incorrect permissions.

💡 Hint

The error mentions 'Address already in use' for the gateway IP.

The error indicates the default Docker bridge network IP range overlaps with an existing network on the host, causing a conflict.

🔀 Workflow

advanced

Sequence to Change Docker Runtime

What is the correct sequence of steps to change the default Docker runtime from runc to nvidia runtime on a Linux host?

A 2,1,3,4

B 1,3,2,4

C 3,1,2,4

D 1,2,3,4

💡 Hint

Configuration changes require daemon restart before verification.

First, configure the daemon.json file, then restart Docker to apply changes, verify the runtime is listed, and finally run containers using the new runtime.

✅ Best Practice

expert

Best Practice for Docker Engine Security Hardening

Which of the following is the best practice to minimize security risks when running Docker Engine in production?

A Run Docker Engine as root user to ensure full access to system resources.

B Disable Docker Content Trust to speed up image pulls.

C Use user namespaces to map container users to non-root host users.

D Allow all containers to run with privileged mode for compatibility.

💡 Hint

Think about isolating container privileges from the host system.

Using user namespaces maps container root users to non-root users on the host, reducing risk if a container is compromised.

Practice

1. What is the primary role of the Docker engine in container management?

easy

A. It manages and controls containers on your system.

B. It builds the Linux kernel for container support.

C. It acts as a cloud service for container hosting.

D. It provides a graphical interface for container design.

Solution

1. Step 1: Understand Docker engine's function

   The Docker engine is the main software that manages containers, including building, running, and controlling them.

2. Step 2: Compare options with this role

   Only It manages and controls containers on your system. correctly states that Docker engine manages and controls containers on your system.

3. Final Answer:

   It manages and controls containers on your system. -> Option A

4. Quick Check:

   Docker engine = container management [OK]

Hint: Docker engine controls containers, not builds kernels or UI [OK]

Common Mistakes:

• Confusing Docker engine with cloud services
• Thinking Docker engine builds the OS kernel
• Assuming Docker engine is a GUI tool

2. Which of the following is the correct command to check the Docker engine version?

easy

A. docker version

B. docker check-version

C. docker show version

D. docker engine info

Solution

1. Step 1: Recall Docker command syntax for version

   The correct command to check Docker engine version is docker version.

2. Step 2: Validate other options

   Options A, B, and C are not valid Docker commands for checking version.

3. Final Answer:

   docker version -> Option A

4. Quick Check:

   Check version command = docker version [OK]

Hint: Use 'docker version' to see engine version quickly [OK]

Common Mistakes:

• Using incorrect command names like 'docker check-version'
• Confusing 'docker info' with version command
• Trying 'docker engine info' which is invalid

3. What will be the output of the command docker ps when no containers are running?

medium

A. Displays an error saying no containers found

B. Lists all containers including stopped ones

C. Shows an empty list with only headers

D. Starts a new container automatically

Solution

1. Step 1: Understand what 'docker ps' shows

   The command docker ps lists running containers only, showing headers even if none are running.

2. Step 2: Analyze output when no containers run

   When no containers run, it shows an empty list with column headers but no container entries.

3. Final Answer:

   Shows an empty list with only headers -> Option C

4. Quick Check:

   docker ps with no containers = empty list [OK]

Hint: 'docker ps' shows running containers; empty if none run [OK]

Common Mistakes:

• Thinking it lists stopped containers too
• Expecting an error message when no containers run
• Assuming it starts containers automatically

4. You run docker run -d nginx but get an error: "Cannot connect to the Docker daemon." What is the likely cause?

medium

A. The container is already running.

B. The nginx image does not exist on Docker Hub.

C. The command syntax is incorrect.

D. Docker daemon is not running on the system.

Solution

1. Step 1: Understand the error message

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

2. Step 2: Check other options

   Image absence or syntax errors cause different errors; container running does not cause daemon connection errors.

3. Final Answer:

   Docker daemon is not running on the system. -> Option D

4. Quick Check:

   Daemon connection error = daemon not running [OK]

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

Common Mistakes:

• Assuming image absence causes daemon errors
• Blaming command syntax for daemon connection issues
• Thinking container running causes this error

5. You want to run a container using a runtime other than the default Docker runtime. Which command option lets you specify the runtime?

hard

A. docker run --use-runtime=<runtime_name> <image>

B. docker run --runtime=<runtime_name> <image>

C. docker run --engine=<runtime_name> <image>

D. docker run --container-runtime=<runtime_name> <image>

Solution

1. Step 1: Recall Docker run options for runtime

   The correct option to specify a runtime is --runtime followed by the runtime name.

2. Step 2: Verify other options

   Options A, B, and D are not valid Docker CLI options for runtime selection.

3. Final Answer:

   docker run --runtime=<runtime_name> <image> -> Option B

4. Quick Check:

   Specify runtime with --runtime option [OK]

Hint: Use '--runtime' option to select container runtime [OK]

Common Mistakes:

• Using incorrect flags like --engine or --use-runtime
• Confusing runtime with engine options
• Omitting the runtime option entirely