Running containers as non-root in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to switch to a non-root user named 'appuser'.

Docker

USER [1]

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

Aroot

Bguest

Cadmin

Dappuser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root' runs the container as root, which is not secure.

The USER instruction sets the user for running commands inside the container. Using appuser runs the container as a non-root user.

2fill in blank

medium

Complete the Dockerfile command to create a non-root user named 'appuser' with home directory.

Docker

RUN adduser --disabled-password --gecos '' [1]

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

Aappuser

Broot

Cadmin

Dguest

Attempts:

3 left

💡 Hint

Common Mistakes

Creating 'root' or 'admin' users defeats the purpose of non-root containers.

The adduser command creates a new user. Here, 'appuser' is created without a password for safer container use.

3fill in blank

hard

Fix the error in the Dockerfile line to set ownership of /app directory to the non-root user 'appuser'.

Docker

RUN chown -R [1] /app

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

Aappuser:appuser

Bguest:guest

Croot:root

Dadmin:admin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root:root' keeps root ownership, causing permission issues for non-root user.

Changing ownership to 'appuser:appuser' ensures the non-root user can access and modify /app safely.

4fill in blank

hard

Fill both blanks to run the container with user ID 1001 and group ID 1001.

Docker

docker run -u [1]:[2] myapp

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

A1001

Broot

C1000

Dappuser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root' or usernames instead of numeric IDs causes errors.

The -u flag runs the container as a specific user and group ID. Using '1001:1001' runs as non-root user and group.

5fill in blank

hard

Fill all three blanks to create a Dockerfile snippet that creates a non-root user 'appuser', sets ownership of /app, and switches to that user.

Docker

RUN adduser --disabled-password --gecos '' [1] && \
    chown -R [2] /app
USER [3]

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

Aappuser

Broot

Cappuser:appuser

Dadmin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root' or 'admin' instead of 'appuser' defeats non-root purpose.

This snippet creates 'appuser', changes ownership of /app to 'appuser:appuser', and switches to 'appuser' to run the container safely as non-root.

Practice

1. Why is it recommended to run Docker containers as a non-root user?

easy

A. To enable root access inside the container

B. To make containers run faster

C. To improve security by limiting container permissions

D. To allow containers to access all host files

Solution

1. Step 1: Understand container user privileges

   Running containers as root gives full permissions inside the container, which can be risky if the container is compromised.

2. Step 2: Recognize security benefits of non-root users

   Using a non-root user limits what the container can do, reducing potential damage from attacks.

3. Final Answer:

   To improve security by limiting container permissions -> Option C

4. Quick Check:

   Security = D [OK]

Hint: Non-root users limit permissions, improving security [OK]

Common Mistakes:

• Thinking non-root users make containers faster
• Believing root is needed for all container tasks
• Confusing root access with better security

2. Which Dockerfile instruction sets the user to run the container as non-root?

easy

A. RUN

B. CMD

C. ENTRYPOINT

D. USER

Solution

1. Step 1: Identify Dockerfile instructions for user control

   The USER instruction sets the user and group for running commands inside the container.

2. Step 2: Differentiate from other instructions

   RUN executes commands during build, CMD and ENTRYPOINT define container start commands but do not set user.

3. Final Answer:

   USER -> Option D

4. Quick Check:

   Set user = USER [OK]

Hint: USER sets container run user in Dockerfile [OK]

Common Mistakes:

• Using RUN to set user (it runs commands)
• Confusing CMD or ENTRYPOINT with user setting
• Omitting USER instruction for non-root

3. What is the effect of running this command?

docker run --user 1001 alpine id

medium

A. Shows the user ID 1001 inside the container

B. Runs the container as root user

C. Fails because user 1001 does not exist

D. Shows the container's hostname

Solution

1. Step 1: Understand the --user flag

   The --user 1001 option runs the container process as user ID 1001 instead of root.

2. Step 2: Analyze the command output

   The id command inside the container prints the current user info, so it will show UID=1001.

3. Final Answer:

   Shows the user ID 1001 inside the container -> Option A

4. Quick Check:

   --user 1001 + id = UID 1001 [OK]

Hint: --user sets UID; id shows current user info [OK]

Common Mistakes:

• Assuming container runs as root despite --user
• Thinking id shows hostname
• Believing command fails if user not in /etc/passwd

4. You wrote this Dockerfile snippet:

FROM alpine
USER appuser
CMD ["sh"]

But when running the container, you get a permission denied error. What is the likely cause?

medium

A. The CMD syntax is incorrect

B. The user 'appuser' does not exist in the image

C. The base image alpine does not support USER

D. The container needs root to run sh

Solution

1. Step 1: Check if user exists in image

   The USER appuser requires that 'appuser' is created in the image, otherwise permission errors occur.

2. Step 2: Verify other options

   CMD syntax is correct, alpine supports USER, and sh can run as non-root if permissions allow.

3. Final Answer:

   The user 'appuser' does not exist in the image -> Option B

4. Quick Check:

   User must exist = B [OK]

Hint: Create user before USER instruction to avoid permission errors [OK]

Common Mistakes:

• Assuming USER works without creating user
• Blaming CMD syntax for permission issues
• Thinking alpine disallows USER instruction

5. You want to run a container as a non-root user with UID 2000 and GID 3000, but the image has no such user. Which is the best way to do this securely?

hard

A. Add a user with UID 2000 and GID 3000 in the Dockerfile, then use USER

B. Run the container as root and change permissions inside

C. Use --user 2000:3000 without creating the user in the image

D. Modify the host system to add user 2000

Solution

1. Step 1: Understand user creation in Dockerfile

   To run as a specific non-root user, create that user and group inside the image with matching UID and GID.

2. Step 2: Use USER instruction to switch user

   After creating the user, use the USER instruction to run container processes as that user securely.

3. Step 3: Evaluate other options

   Running as root is insecure, using --user without user in image may cause permission issues, and modifying host users is unrelated.

4. Final Answer:

   Add a user with UID 2000 and GID 3000 in the Dockerfile, then use USER -> Option A

5. Quick Check:

   Create user + USER = secure non-root run [OK]

Hint: Create user in Dockerfile before USER for secure non-root run [OK]

Common Mistakes:

• Running as root to avoid user setup
• Using --user without user in image causing errors
• Changing host users instead of container users