Mounting read-only volumes in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of this Docker run command?

Consider the command below that runs a Docker container with a volume mounted as read-only. What will be the output when the container tries to write to the mounted directory?

Docker

docker run --rm -v /host/data:/container/data:ro alpine sh -c "echo 'test' > /container/data/file.txt"

A The command fails with a 'Read-only file system' error.

B The command succeeds and writes 'test' to /container/data/file.txt inside the container.

C The container ignores the read-only flag and writes the file successfully.

D The container crashes immediately without any error message.

💡 Hint

Think about what the ':ro' flag means when mounting volumes in Docker.

Mounting a volume with ':ro' makes the mounted directory read-only inside the container. Any attempt to write to it will fail with a 'Read-only file system' error.

🧠 Conceptual

intermediate

Why use read-only volume mounts in Docker?

Which of the following is the best reason to mount a volume as read-only in a Docker container?

A To increase the container's CPU usage.

B To allow the container to write logs to the host system.

C To enable the container to update configuration files on the host.

D To prevent the container from modifying host files and improve security.

💡 Hint

Think about protecting host data from accidental or malicious changes.

Mounting volumes as read-only prevents containers from changing host files, which enhances security and stability.

❓ Configuration

advanced

Identify the correct Docker Compose volume mount for read-only access

Given this Docker Compose service snippet, which option correctly mounts the host directory '/app/config' as read-only inside the container at '/config'?

Docker

services:
  app:
    image: myapp:latest
    volumes:

A - /app/config:/config:ro

B - /app/config:/config:rw

C - /app/config:/config

D - /app/config:/config:read-only

💡 Hint

Docker Compose uses the same volume mount syntax as Docker CLI for read-only flag.

The ':ro' suffix marks the volume as read-only. ':rw' is read-write (default), and 'read-only' is invalid syntax.

❓ Troubleshoot

advanced

Why does this container fail to write to a mounted volume despite no ':ro' flag?

A developer runs this command: docker run --rm -v /host/data:/data alpine sh -c "echo 'hello' > /data/file.txt" But the command fails with 'Read-only file system'. What is the most likely cause?

A The container image 'alpine' does not support writing to volumes.

B The host directory '/host/data' is mounted as read-only by the host OS permissions.

C The volume mount syntax is incorrect and defaults to read-only.

D The Docker daemon is running in read-only mode.

💡 Hint

Check the permissions of the host directory being mounted.

If the host directory has read-only permissions, the container cannot write to it even if the volume is mounted without ':ro'.

🔀 Workflow

expert

Order the steps to safely mount a read-only volume in Docker for a production app

Arrange the following steps in the correct order to mount a read-only volume for a Docker container running a production app.

A 4,2,1,3

B 1,4,2,3

C 4,1,2,3

D 2,4,1,3

💡 Hint

Think about preparing data and permissions before running the container.

First prepare data (4), then set permissions (1), run container with ':ro' (2), finally verify (3).

Practice

1. What does adding :ro at the end of a Docker volume mount do?

easy

A. Mounts the volume with root user permissions

B. Mounts the volume as read-write, allowing container writes

C. Removes the volume after container stops

D. Mounts the volume as read-only, preventing container writes

Solution

1. Step 1: Understand Docker volume mount options

   Docker allows mounting volumes with options like :ro or :rw to control access.

2. Step 2: Identify the effect of :ro

   The :ro option makes the mounted volume read-only inside the container, so the container cannot modify the data.

3. Final Answer:

   Mounts the volume as read-only, preventing container writes -> Option D

4. Quick Check:

   :ro means read-only mount [OK]

Hint: Remember 'ro' means read-only, no writes allowed [OK]

Common Mistakes:

• Confusing :ro with :rw
• Thinking :ro deletes volume after use
• Assuming :ro changes user permissions

2. Which of the following is the correct syntax to mount a host directory /data as a read-only volume inside a container at /app/data?

easy

A. docker run -v /data:/app/data:rw myimage

B. docker run -v /data:/app/data:read-only myimage

C. docker run -v /data:/app/data:ro myimage

D. docker run -v /data:/app/data:readonly myimage

Solution

1. Step 1: Recall Docker volume mount syntax

   The syntax is -v host_path:container_path:options, where options can be ro or rw.

2. Step 2: Identify correct read-only option

   The correct option for read-only is ro. Options like read-only or readonly are invalid.

3. Final Answer:

   docker run -v /data:/app/data:ro myimage -> Option C

4. Quick Check:

   Use :ro for read-only mount [OK]

Hint: Use ':ro' exactly for read-only volume mounts [OK]

Common Mistakes:

• Using ':rw' instead of ':ro' for read-only
• Writing 'readonly' or 'read-only' instead of 'ro'
• Omitting the option entirely

3. Given the command:

docker run -v /host/config:/app/config:ro myapp

What will happen if the container tries to write a file inside /app/config?

medium

A. The write will fail with a permission error

B. The container will crash immediately

C. The write will succeed and file will be saved

D. Docker will remount the volume as read-write automatically

Solution

1. Step 1: Understand the effect of :ro mount

   The volume is mounted read-only, so the container cannot modify files in /app/config.

2. Step 2: Predict container behavior on write attempt

   When the container tries to write, the OS denies permission, causing a write failure with an error.

3. Final Answer:

   The write will fail with a permission error -> Option A

4. Quick Check:

   Read-only mount blocks writes with permission error [OK]

Hint: Read-only volumes block writes, causing permission errors [OK]

Common Mistakes:

• Assuming writes succeed on read-only mounts
• Thinking Docker remounts volumes automatically
• Believing container crashes on write failure

4. You run this command:

docker run -v /data:/app/data myimage

You want to make the volume read-only but accidentally write :ro as :or. What will happen?

medium

A. Docker will mount the volume as read-only anyway

B. Docker will throw an error and refuse to start the container

C. The volume will be mounted as read-write

D. The container will start but fail when writing to the volume

Solution

1. Step 1: Check the mount option syntax

   The option :ro is valid for read-only. :or is invalid and not recognized by Docker.

2. Step 2: Understand Docker's response to invalid options

   Docker will detect the invalid option and refuse to start the container, showing an error.

3. Final Answer:

   Docker will throw an error and refuse to start the container -> Option B

4. Quick Check:

   Invalid mount options cause Docker errors [OK]

Hint: Check option spelling carefully; invalid options cause errors [OK]

Common Mistakes:

• Assuming Docker ignores invalid mount options
• Thinking volume mounts as read-only despite typo
• Believing container starts but fails later

5. You want to share a configuration directory /etc/config with multiple containers but prevent them from modifying it. Which command correctly mounts it read-only and ensures the container user cannot override this?

hard

A. docker run -v /etc/config:/config:ro myapp

B. docker run --read-only -v /etc/config:/config myapp

C. docker run -v /etc/config:/config:rw --user 1000 myapp

D. docker run -v /etc/config:/config:rw myapp

Solution

1. Step 1: Understand read-only volume mount

   Mounting with :ro makes the volume read-only inside the container, preventing writes regardless of user.

2. Step 2: Evaluate other options

   docker run --read-only -v /etc/config:/config myapp makes the entire container filesystem read-only, which is overly restrictive. docker run -v /etc/config:/config:rw --user 1000 myapp adds user but does not prevent writes since volume is read-write. docker run -v /etc/config:/config:rw myapp mounts read-write, allowing changes.

3. Final Answer:

   docker run -v /etc/config:/config:ro myapp -> Option A

4. Quick Check:

   Use :ro to share read-only volumes safely [OK]

Hint: Use ':ro' to protect shared data from container writes [OK]

Common Mistakes:

• Confusing container read-only flag with volume read-only
• Mounting volume as read-write by mistake
• Thinking user ID affects volume write permissions