How to Secure Docker Containers with Best Practices

Docker containers simplify the development and deployment of applications, but they also introduce security challenges. This tutorial will walk you through five essential best practices to secure your Docker containers effectively.

Prerequisites

 
To follow along:

• You should have Docker installed.
• You should be comfortable with Docker commands for building images and creating Dockerfiles for your applications.

1. Use Official Base Images

 
Official images are maintained by trusted sources and are regularly updated with security patches, reducing the likelihood of vulnerabilities.

Always start your Dockerfile with an official image from Docker Hub.

Regularly monitor the official repositories for updates to your base images and rebuild your containers as needed.

2. Minimize the Attack Surface

 
The larger your image, the more vulnerabilities it is susceptible to. Reducing the size of your Docker image minimizes the attack surface.

Use minimal base images like alpine, which are significantly smaller and contain fewer (but necessary) packages. Additionally, consider using multi-stage builds to ensure that only the essential components are included in the final image.

Here’s an example Dockerfile that uses multi-stage builds for a Go app:

# Stage 1: Build the application
FROM golang:1.19-alpine AS builder

# Set the working directory in the builder container
WORKDIR /app

# Copy the Go source code
COPY . .

# Build the Go application
RUN go build -o myapp

# Stage 2: Create the minimal final image
FROM alpine:3.18

# Set the working directory
WORKDIR /app

# Copy the binary from the build stage
COPY --from=builder /app/myapp .

# Run the application
CMD ["./myapp"]

It also helps to regularly audit your Dockerfiles to remove unnecessary tools, files, and dependencies. This not only reduces the image size but also eliminates potential vulnerabilities.

3. Run as a Non-Root User

 
By default, you’ll run Docker containers as the root user, which can be dangerous if the container is compromised. Running as a non-root user mitigates the risk of privilege escalation attacks and limits the damage that an attacker can inflict.

Create a dedicated user in your Dockerfile and switch to it using the USER instruction:

RUN useradd -r -s /bin/false appuser
USER appuser

Regularly verify that your container does not inadvertently regain root privileges during operation, and ensure that all files and directories have appropriate permissions.

4. Use Docker Secrets for Sensitive Data

 
Hardcoding sensitive data like passwords, API keys, and tokens in your Dockerfile or environment variables can lead to security breaches. Docker secrets provide a secure way to manage and access sensitive information.

Docker secrets are stored in encrypted form and can be accessed by containers running as services in Docker Swarm. Use them to store and manage sensitive data securely.

Here’s how to create and manage secrets in a Docker Swarm environment:

1. First, create your secret using the Docker CLI:

$ echo "my-secret-password" | docker secret create db_password -

2. For local development, you can store secrets in files:

# ./secrets/db_password.txt
my-secret-password

Now, let’s look at how your application can access these secrets. When Docker mounts a secret, it becomes available to the container at `/run/secrets/secret_name&gt`. Here’s a Python example of how to read it:

def get_secret(secret_name):
    try:
        with open(f'/run/secrets/secret_name', 'r') as secret_file:
            return secret_file.read().strip()
    except IOError:
        return None

# Use the secret in your application
db_password = get_secret('db_password')
api_key = get_secret('api_key')

 
The secrets will be mounted at runtime, and your application can access them as regular files. This provides a secure way to handle sensitive data without exposing it in your application code or Docker configuration.

5. Enable Docker Content Trust

 
Docker Content Trust (DCT) ensures that the images you pull are signed and verified, preventing the use of tampered or malicious images.

This ensures that only signed images are used, providing an additional layer of security.

Read Content trust in Docker to learn more about enabling and using DCT.

Remember, container security is an ongoing process, not a one-time setup. Regularly audit your container configurations, monitor for unusual behavior, and keep up with the latest security best practices in the container ecosystem.

Additional Resources

 
To learn more, check the following:

Bala Priya C is a developer and technical writer from India. She likes working at the intersection of math, programming, data science, and content creation. Her areas of interest and expertise include DevOps, data science, and natural language processing. She enjoys reading, writing, coding, and coffee! Currently, she’s working on learning and sharing her knowledge with the developer community by authoring tutorials, how-to guides, opinion pieces, and more. Bala also creates engaging resource overviews and coding tutorials.