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).