Running containers as non-root in Docker - Mini Project: Build & Apply

Running Containers as Non-Root User

📖 Scenario: You are deploying a simple web application using Docker. For security reasons, you want the container to run as a non-root user instead of the default root user.

🎯 Goal: Learn how to create a Dockerfile that sets up a non-root user and runs the container with that user.

📋 What You'll Learn

Create a Dockerfile with a base image

Add a non-root user to the Dockerfile

Set the container to run as the non-root user

Verify the container runs as the non-root user

💡 Why This Matters

🌍 Real World

Running containers as non-root users improves security by limiting permissions inside the container, reducing risks if the container is compromised.

💼 Career

Many DevOps and security roles require knowledge of container security best practices, including running containers as non-root users.

1

Create a Dockerfile with a base image

Create a file named Dockerfile and write a line to use the python:3.12-slim base image.

Docker

# Write the FROM instruction to use python:3.12-slim
# Your code here

Hint

The FROM instruction sets the base image for your Dockerfile.

2

Add a non-root user

Add lines to the Dockerfile to create a new user named appuser with user ID 1001 and create a home directory for this user.

Docker

FROM python:3.12-slim
# Add a user named appuser with UID 1001
# Your code here

Hint

Use RUN useradd -m -u 1001 appuser to add the user with a home directory.

3

Set the container to run as the non-root user

Add a line to the Dockerfile to switch the user to appuser so the container runs as this user.

Docker

FROM python:3.12-slim
RUN useradd -m -u 1001 appuser
# Set the user to appuser
# Your code here

Hint

The USER instruction sets the user for running the container.

4

Build and run the container to verify the user

Write the Docker commands to build the image named nonrootapp from the current directory and run a container that prints the current user with whoami.

Docker

FROM python:3.12-slim
RUN useradd -m -u 1001 appuser
USER appuser
# Build the image and run the container to print the user
# Your code here

Hint

Use docker build -t nonrootapp . to build and docker run --rm nonrootapp whoami to check the user.

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