Mounting read-only volumes in Docker - Mini Project: Build & Apply

Mounting Read-Only Volumes in Docker

📖 Scenario: You are working on a Docker container that needs to access some files from your host machine. To keep the container safe and prevent accidental changes, you want to mount the files as read-only.

🎯 Goal: Learn how to mount a host directory into a Docker container as a read-only volume.

📋 What You'll Learn

Create a Docker volume mount command with a host directory

Add a read-only flag to the volume mount

Run a container that uses the read-only volume

Verify the volume is mounted as read-only inside the container

💡 Why This Matters

🌍 Real World

Mounting read-only volumes is useful when you want to share configuration files or data with containers without risking accidental changes.

💼 Career

Understanding volume mounts and permissions is essential for container management and deployment in DevOps roles.

1

Create a host directory with a file

Create a directory called mydata in your current folder and inside it create a file named info.txt with the text Hello Docker.

Docker

# Create the directory and file on your host machine
# Your code here

Hint

Use mkdir -p mydata to create the directory and echo to write text to the file.

2

Write the Docker run command with volume mount

Write a docker run command that starts an alpine container and mounts the host directory mydata to /data inside the container.

Docker

mkdir -p mydata
echo "Hello Docker" > mydata/info.txt
# Write the docker run command with volume mount here
# Your code here

Hint

Use -v $(pwd)/mydata:/data to mount the directory.

3

Add the read-only flag to the volume mount

Modify the docker run command to mount the volume as read-only by adding :ro at the end of the volume mount.

Docker

mkdir -p mydata
echo "Hello Docker" > mydata/info.txt
docker run --rm -it -v $(pwd)/mydata:/data alpine sh
# Add :ro to the volume mount to make it read-only
# Your code here

Hint

Add :ro after the container path in the volume mount to make it read-only.

4

Verify the volume is read-only inside the container

Inside the running container, try to create a new file /data/newfile.txt. Use the command touch /data/newfile.txt and observe the output. Then print the contents of /data/info.txt using cat /data/info.txt.

Docker

mkdir -p mydata
echo "Hello Docker" > mydata/info.txt
docker run --rm -it -v $(pwd)/mydata:/data:ro alpine sh
# Inside container, try to create a file and read the existing file
# Your code here

Hint

The touch command should fail because the volume is read-only. The cat command should display Hello Docker.

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