Common container startup failures in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to start a Docker container in detached mode.

Docker

docker run [1] nginx

Drag options to blanks, or click blank then click option'

A--name

B--rm

C-p

D-d

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--rm' which removes the container after it stops.

Using '-p' which is for port mapping.

Using '--name' which names the container.

The '-d' flag runs the container in detached mode, allowing it to run in the background.

2fill in blank

medium

Complete the command to map port 8080 on the host to port 80 in the container.

Docker

docker run -p [1]:80 nginx

Drag options to blanks, or click blank then click option'

A8080

B443

C80

D3000

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the ports and using '80:8080' which maps the wrong way.

Using port 443 which is for HTTPS by default.

Using port 3000 which is unrelated here.

Port 8080 on the host is mapped to port 80 inside the container using '-p 8080:80'.

3fill in blank

hard

Fix the error in the Dockerfile to set the working directory.

Docker

WORKDIR [1]/app

Drag options to blanks, or click blank then click option'

Ausr/local

B/usr/local

C/usr/local/

Dusr/local/

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the leading slash causing path errors.

Adding trailing slashes inconsistently.

Using relative paths instead of absolute.

The WORKDIR path must be absolute, starting with a slash '/' to avoid errors.

4fill in blank

hard

Fill both blanks to create a Docker volume mount for persistent data.

Docker

docker run -v [1]:[2] nginx

Drag options to blanks, or click blank then click option'

A/host/data

B/container/data

C/data

D/var/lib

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping host and container paths.

Using relative paths instead of absolute.

Using system directories that may cause permission issues.

The volume mount syntax is '-v host_path:container_path'. Here, '/host/data' is the host directory and '/container/data' is the container directory.

5fill in blank

hard

Fill all three blanks to write a Dockerfile command that copies files and sets environment variables.

Docker

COPY [1] [2]
ENV [3]=production

Drag options to blanks, or click blank then click option'

Asrc/

B/app/

CNODE_ENV

Ddist/

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong source or destination paths.

Forgetting to set environment variables correctly.

Mixing up COPY and ADD commands.

The COPY command copies 'src/' directory to '/app/' in the container. ENV sets the environment variable 'NODE_ENV' to 'production'.

Practice

1. What is a common reason a Docker container might fail to start immediately after running docker run?

easy

A. The container tries to use a port already in use on the host

B. The Docker daemon is running

C. The container image is very large

D. The host has enough free disk space

Solution

1. Step 1: Understand port conflicts in Docker

   If a container tries to bind to a port already used by another process on the host, it cannot start because the port is busy.

2. Step 2: Check other options for relevance

   Options B, C, and D do not prevent container startup immediately; only port conflict causes failure here.

3. Final Answer:

   The container tries to use a port already in use on the host -> Option A

4. Quick Check:

   Port conflict = container startup failure [OK]

Hint: Check if the port is free before starting container [OK]

Common Mistakes:

• Assuming large image size stops container start
• Thinking Docker daemon running causes failure
• Ignoring port conflicts

2. Which of the following docker run commands will correctly set an environment variable named APP_ENV to production?

easy

A. docker run --env APP_ENV production myimage

B. docker run -e APP_ENV production myimage

C. docker run -env APP_ENV=production myimage

D. docker run -e APP_ENV=production myimage

Solution

1. Step 1: Recall correct syntax for environment variables

   The correct flag is -e or --env followed by VAR=VALUE without spaces.

2. Step 2: Analyze each option

   docker run -e APP_ENV=production myimage uses correct syntax: -e APP_ENV=production. Options A, C, and D have syntax errors or missing '=' sign.

3. Final Answer:

   docker run -e APP_ENV=production myimage -> Option D

4. Quick Check:

   Use -e VAR=VALUE for env vars [OK]

Hint: Use -e VAR=VALUE with no spaces for env vars [OK]

Common Mistakes:

• Omitting '=' between variable and value
• Using incorrect flag like -env
• Adding spaces between variable and value

3. Given this command:

docker run -p 8080:80 nginx

What will happen if port 8080 on the host is already used by another service?

medium

A. Docker will automatically choose a different host port

B. The container will fail to start with a port binding error

C. The container will start normally and ignore the port conflict

D. The container will start but the service inside will not work

Solution

1. Step 1: Understand port binding behavior

   If the host port (8080) is already in use, Docker cannot bind the container port to it, causing failure.

2. Step 2: Evaluate other options

   Docker does not auto-select ports unless specified with -P. The container won't start ignoring conflicts or silently fail.

3. Final Answer:

   The container will fail to start with a port binding error -> Option B

4. Quick Check:

   Port in use causes start failure [OK]

Hint: Check host ports before binding with -p [OK]

Common Mistakes:

• Assuming Docker auto-changes ports
• Thinking container starts ignoring conflicts
• Believing service inside fails but container runs

4. You run a container with docker run -v /host/data:/app/data myimage but the container fails to start. What is the most likely cause?

medium

A. The image name is incorrect

B. The container does not have enough CPU resources

C. The host directory /host/data does not exist or has wrong permissions

D. The Docker daemon is not running

Solution

1. Step 1: Understand volume mount requirements

   Docker requires the host directory to exist and be accessible for volume mounts; otherwise, container startup fails.

2. Step 2: Check other options

   Incorrect image name causes image pull errors, not volume mount failure. CPU or daemon issues cause different errors.

3. Final Answer:

   The host directory /host/data does not exist or has wrong permissions -> Option C

4. Quick Check:

   Volume mount fails if host path missing [OK]

Hint: Verify host path exists and permissions before mounting [OK]

Common Mistakes:

• Ignoring host directory existence
• Confusing image name errors with volume errors
• Assuming CPU limits cause mount failures

5. You have a container that fails immediately after starting. You run docker logs container_id and see an error about a missing environment variable DB_HOST. How should you fix this?

hard

A. Add -e DB_HOST=value to the docker run command

B. Increase the container's CPU and memory limits

C. Change the container image to a different one

D. Restart the container without changes

Solution

1. Step 1: Interpret the error message from logs

   The logs indicate the container expects an environment variable DB_HOST which is missing, causing failure.

2. Step 2: Provide the missing environment variable

   Use -e DB_HOST=value in the docker run command to supply the required variable.

3. Step 3: Understand why other options fail

   Restarting without changes won't fix missing vars. Changing image or resources doesn't address environment variables.

4. Final Answer:

   Add -e DB_HOST=value to the docker run command -> Option A

5. Quick Check:

   Missing env var fixed by -e flag [OK]

Hint: Use docker logs to find missing env vars, then add -e [OK]

Common Mistakes:

• Restarting without fixing environment variables
• Changing image unnecessarily
• Increasing resources instead of fixing config