Running a container with docker run - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker run with port mapping

What will be the output when running this command?

docker run -d -p 8080:80 nginx

Assume no other containers are running and Docker is installed correctly.

A Starts a detached nginx container mapping host port 8080 to container port 80 and outputs the container ID

B Starts nginx container in foreground and outputs nginx logs

C Fails with error: port 8080 already in use

D Starts a container but does not map any ports

💡 Hint

The -d flag runs the container in detached mode, and -p maps ports.

The command runs nginx detached (-d) and maps host port 8080 to container port 80 (-p 8080:80). Docker outputs the container ID on success.

🧠 Conceptual

intermediate

Purpose of --rm flag in docker run

What does the --rm flag do when used with docker run?

A Automatically removes the container after it stops

B Restarts the container automatically if it crashes

C Removes the image after container creation

D Runs the container in read-only mode

💡 Hint

Think about container cleanup after execution.

The --rm flag tells Docker to delete the container once it stops running, keeping the system clean.

❓ Troubleshoot

advanced

Troubleshooting docker run with volume mount error

You run this command:

docker run -v /host/data:/container/data busybox ls /container/data

But get an empty output even though /host/data has files. What is the likely cause?

A The container path /container/data is incorrect and should be /data

B The busybox image does not support volume mounts

C /host/data path does not exist or is empty on the host

D Docker daemon is not running

💡 Hint

Check the host directory before running the container.

If the host directory is empty or missing, the container will show empty contents at the mount point.

🔀 Workflow

advanced

Correct order to run a container with environment variables and port mapping

Arrange these steps in the correct order to run a container with environment variable MY_VAR=hello and map port 5000 to 80:

A 1,3,2,4

B 3,1,2,4

C 2,1,3,4

D 1,2,3,4

💡 Hint

You must have the image before running the container.

First pull the image, then run with options, check running containers, then verify env variable inside.

✅ Best Practice

expert

Best practice for running containers with sensitive environment variables

Which option is the best practice to securely pass sensitive environment variables like passwords to a container using docker run?

A Use docker run -e PASSWORD=secret directly in the command line

B Store sensitive variables in a file and use --env-file option with docker run

C Hardcode sensitive variables inside the Dockerfile ENV instruction

D Pass sensitive variables as command arguments inside the container

💡 Hint

Think about avoiding exposure in command history or image layers.

Using --env-file keeps sensitive data out of command history and Dockerfile, improving security.

Practice

1. What does the docker run command do?

easy

A. Starts a new container from a specified image

B. Builds a new Docker image from a Dockerfile

C. Stops a running Docker container

D. Deletes an existing Docker image

Solution

1. Step 1: Understand the purpose of docker run

   The docker run command is used to start a new container based on a Docker image.

2. Step 2: Differentiate from other Docker commands

   Commands like docker build create images, docker stop stops containers, and docker rmi removes images, so they are not correct here.

3. Final Answer:

   Starts a new container from a specified image -> Option A

4. Quick Check:

   docker run = start container [OK]

Hint: Remember: run = start container from image [OK]

Common Mistakes:

• Confusing 'run' with 'build' command
• Thinking 'run' deletes images
• Mixing 'run' with stopping containers

2. Which of the following is the correct syntax to run an interactive container with a bash shell using the image ubuntu?

easy

A. docker run ubuntu -it bash

B. docker run -d ubuntu bash

C. docker run -it ubuntu bash

D. docker run ubuntu -d -it bash

Solution

1. Step 1: Identify flags for interactive mode

   The -it flag is used to run a container interactively with a terminal attached.

2. Step 2: Check command order and flags

   The correct syntax places flags immediately after docker run, then the image name, then the command to run inside the container.

3. Final Answer:

   docker run -it ubuntu bash -> Option C

4. Quick Check:

   Flags before image, -it for interactive [OK]

Hint: Put flags right after 'docker run', then image, then command [OK]

Common Mistakes:

• Placing flags after image name
• Using -d for interactive shell
• Mixing order of command and flags

3. What will be the output of the following command?

docker run --rm alpine echo Hello World

medium

A. Hello World

B. Container ID printed

C. Error: alpine image not found

D. No output, container runs in background

Solution

1. Step 1: Understand the command components

   docker run --rm alpine echo Hello World runs the echo Hello World command inside an Alpine container and removes the container after it finishes.

2. Step 2: Predict the output

   The command prints Hello World to the terminal, then the container stops and is removed automatically.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   Run with echo prints text, --rm cleans up [OK]

Hint: Echo prints text, --rm removes container after run [OK]

Common Mistakes:

• Expecting container ID output
• Thinking container stays running
• Assuming image is missing

4. You run docker run -p 8080:80 nginx but cannot access the web server on port 8080. What is a likely cause?

medium

A. Port mapping syntax is incorrect

B. The container is not running

C. You forgot to add -d to run detached

D. Port 80 inside container is not exposed

Solution

1. Step 1: Check container running status

   If the container is not running, port mapping won't work and you can't access the server.

2. Step 2: Verify port exposure and syntax

   By default, nginx listens on port 80 and the syntax -p 8080:80 is correct. -d is optional but recommended for background.

3. Final Answer:

   The container is not running -> Option B

4. Quick Check:

   Container must run for port mapping to work [OK]

Hint: Check if container is running before checking ports [OK]

Common Mistakes:

• Assuming port 80 is not exposed by nginx
• Thinking -d is required for port mapping
• Misunderstanding port mapping syntax

5. You want to run a container from the image myapp that needs environment variables API_KEY and DEBUG set, and expose port 5000 on the host to port 80 in the container. Which command is correct?

hard

A. docker run -e API_KEY=123 DEBUG=true -p 5000:80 myapp

B. docker run -p 80:5000 -e API_KEY=123 -e DEBUG=true myapp

C. docker run myapp -e API_KEY=123 -e DEBUG=true -p 5000:80

D. docker run -e API_KEY=123 -e DEBUG=true -p 5000:80 myapp

Solution

1. Step 1: Set environment variables correctly

   Use -e VAR=value for each environment variable before the image name.

2. Step 2: Map ports correctly

   Host port 5000 maps to container port 80 with -p 5000:80. The order is flags, then image name.

3. Step 3: Verify command order and syntax

   Options must come before the image name. docker run -p 80:5000 -e API_KEY=123 -e DEBUG=true myapp reverses ports, C places options after image, D misses -e for second variable.

4. Final Answer:

   docker run -e API_KEY=123 -e DEBUG=true -p 5000:80 myapp -> Option D

5. Quick Check:

   Flags before image, correct port and env syntax [OK]

Hint: Put all flags before image; use -e for each env var [OK]

Common Mistakes:

• Placing options after image name
• Reversing port mapping order
• Missing -e for environment variables