Running containers as non-root in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Dockerfile command?

Consider this Dockerfile snippet that sets a non-root user:

FROM alpine:latest
RUN adduser -D appuser
USER appuser
RUN whoami

What will be the output of the whoami command inside the container?

Docker

FROM alpine:latest
RUN adduser -D appuser
USER appuser
RUN whoami

A appuser

B root

C nobody

D docker

💡 Hint

The USER instruction changes the user for following commands.

The USER instruction switches the user to 'appuser'. So the 'whoami' command runs as 'appuser' and outputs 'appuser'.

❓ Configuration

intermediate

Which Dockerfile snippet correctly runs a process as non-root user?

You want to run your application inside a container as a user named 'appuser' with UID 1001. Which Dockerfile snippet correctly creates this user and runs the app as that user?

A

RUN adduser -u 1001 appuser
USER root
CMD ["./app"]

B

RUN useradd -m -u 1001 appuser
USER root
CMD ["./app"]

C

RUN adduser appuser
USER 1001
CMD ["./app"]

D

RUN adduser -u 1001 appuser
USER appuser
CMD ["./app"]

💡 Hint

The USER instruction sets the user for running commands and the app.

Option D creates the user with UID 1001, switches to that user, and runs the app as that user. Other options either keep root or use wrong USER syntax.

❓ Troubleshoot

advanced

Why does this container fail to start when running as non-root?

You have this Dockerfile snippet:

FROM node:18
RUN useradd -m appuser
USER appuser
CMD ["node", "app.js"]

The container fails with a permission error when trying to access /app directory. What is the most likely cause?

A The CMD syntax is invalid for non-root users.

B The /app directory is owned by root and appuser has no permission to access it.

C The USER instruction must be root to run node apps.

D The useradd command did not create the user correctly.

💡 Hint

Check directory ownership and permissions for the app user.

The appuser cannot access /app if it is owned by root without proper permissions. This causes permission errors when running as non-root.

🔀 Workflow

advanced

What is the correct order to run these Docker commands to build and run a container as non-root?

Arrange these commands in the correct order to build an image and run a container as a non-root user named 'appuser':

A 3,4,1,2

B 4,3,1,2

C 1,3,4,2

D 3,1,4,2

💡 Hint

You must create the Dockerfile with user setup before building the image.

First create the Dockerfile with USER and user creation, then build the image, then run the container.

✅ Best Practice

expert

Which practice is best for running containers as non-root in production?

Select the best practice for running containers as non-root users in a secure production environment.

A Run containers as random users without setting file permissions.

B Run containers as root but restrict network access with firewall rules.

C Create a dedicated user with a fixed UID and set proper file permissions in the image.

D Use the default root user but disable all container capabilities.

💡 Hint

Security best practices recommend fixed users and permissions.

Creating a dedicated user with fixed UID and setting file permissions ensures predictable security and avoids permission issues.

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