Container Images Demystified: Structure, Security, and Best Practices

What is a container image?

A container image is a lightweight, standalone package that includes everything an application needs to run: code, runtime, libraries, system tools, and configurations. Unlike virtual machines, which require a full operating system, containers share the host system’s kernel but can include a minimal OS layer that’s specific to the application. This makes them faster and more resource-efficient.

This efficiency has made container images essential for cloud computing. Developers can package an application once and run it anywhere, whether that’s on a local machine, a testing server, or a cloud platform. This removes the common “it works on my machine” problem and ensures smooth deployments across different environments.

What are the differences between Docker images and containers?

Understanding the differences between Docker images and containers is the key to working with containerized applications. While people often use these terms interchangeably, they serve distinct roles in container architecture.

• A Docker image is a read-only package that acts as a blueprint for creating containers. Developers can build images from an existing image or a Dockerfile and store them in registries like Docker Hub or Red Hat Quay. These images remain static until they are used to start a container.

• A container is a running instance of a Docker image that comes to life when someone executes it. When it starts, it adds a writable layer on top of the image, which allows for temporary changes during runtime. Since containers run directly on a host machine and share its kernel, they remain lightweight and efficient. To manage containers at scale, orchestration tools like Kubernetes automate deployment, scaling, and networking to ensure seamless operation across cloud environments.

Since containers and Docker images play different roles in containerized applications, understanding their key differences can clarify how they work together. The table below breaks down these distinctions: 

Advanced Container Security Best Practices [Cheat Sheet]

This cheat sheet goes beyond the no-brainer container security best practices and explores advanced techniques that you can put into action ASAP. Use this cheat sheet as a quick reference to ensure you have the proper benchmarks in place to secure your container environments.

Download now

How does container architecture work?

Container architecture relies on these four key components to build, store, and run containers efficiently:

Container images

Think of a container image as a layered blueprint for your application. Each layer represents a change or addition, starting with a base image that includes the operating system and essential libraries.

Key features of container images:

• Layered structure: Base layer + Dependencies + Configurations + Application Code

• Immutability: Once built, the image remains unchanged, ensuring consistency across environments.

• Reusability: Images can be reused across different systems, reducing redundancy.

Creating a container image using a Dockerfile:

Developers create container images using a Dockerfile, which is a script that defines how to build the image:

# Use an official lightweight base image
FROM python:3.9-slim

# Set the working directory inside the container
WORKDIR /app

# Copy application files into the container
COPY . /app

# Install dependencies
RUN pip install -r requirements.txt

# Command to run the application
CMD ["python", "app.py"]

Best practices:

• Use minimal base images like alpine to reduce attack surfaces.

• Regularly update images to patch vulnerabilities.

• Sign and verify images to ensure authenticity before deployment.

Container image registries

A container image registry is a central hub where images are stored and distributed. Developers push images after building them and pull them when deploying applications.

Popular container registries:

• Docker hub: A widely used public registry with a vast collection of images.

• AWS Elastic container registry (ECR): Integrated with AWS cloud services.

• Google container registry: Optimized for Google Cloud deployments.

Managing container images effectively:

• Tagging Images Properly – Assign meaningful tags to track versions:

docker tag my-app:latest my-app:v1.0.0

• Scanning for Vulnerabilities – Use security tools like Trivy or Docker Scout to detect flaws:

trivy image my-app:v1.0.0

• Access Control & Security – Restrict permissions to prevent unauthorized modifications.

Container runtimes

A container runtime is the software that pulls container images from a registry, unpacks them, and runs the application inside an isolated environment. Essentially, it’s what makes containers work.

While Docker has now expanded its capabilities, it’s still the most well-known container runtime and played a major role in popularizing container technology. However, the ecosystem has since grown, and other runtimes now serve different needs. For instance, many Kubernetes environments rely on containerd and CRI-O because they provide a lightweight, optimized way to run containers at scale.

Choosing a runtime depends on the use case you need it for. Docker provides an all-in-one experience with a simple developer workflow, while containerd and CRI-O integrate more efficiently with Kubernetes for large-scale deployments. But regardless of the runtime, the end goal remains the same: ensuring that containers run reliably and securely on any system.

wiz academy

Container Runtime Security

Read more

Union file systems and the copy-on-write mechanism

Containers rely on a union file system (UnionFS) and the copy-on-write (CoW) mechanism to efficiently manage storage and ensure lightweight, fast deployments. Instead of duplicating entire file systems, these technologies enable containers to share common layers while allowing modifications that don’t affect the original image.

• A UnionFS stacks multiple layers into a single unified view. Each layer represents a different stage of the container image, starting with the base layer and adding additional layers for dependencies and application code. These layers remain read-only to ensure consistency across multiple containers running the same image.

• The CoW mechanism comes into play when a container needs to modify a file. Instead of changing the original layer, the system creates a new writable layer on top of the existing ones. The container works with this layer but leaves the underlying image untouched. This approach optimizes storage, speeds up container startup times, and ensures that multiple containers can share the same base image without conflicts.

Together, UnionFS and the CoW mechanism make container images highly efficient, portable, and scalable, reducing redundancy while maintaining flexibility. This is particularly valuable in container orchestration platforms like Kubernetes, where managing multiple containers efficiently is a priority.

The importance of container images

Container images transform how teams deploy and manage applications in the cloud. Their impact goes beyond convenience to offer significant advantages in speed, consistency, scalability, and security. Here are a few of these advantages:

Faster, more efficient deployments

Container images package applications with all their dependencies, which eliminates the need for manual environment setup. This way, developers can move seamlessly from coding to testing and deployment without compatibility issues slowing them down. This streamlined process accelerates software development and reduces downtime.

Consistency across environments

With container images, applications run the same way in every environment—development, testing, or production. This prevents bugs due to configuration differences and ensures that applications perform predictably, whether they’re running on a local machine or in the cloud.

FROM python:3.9-slim
WORKDIR /app
COPY . /app
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

Seamless scalability for microservices

Container images create modern, scalable architectures by breaking applications into smaller, more independent services. This flexibility makes it easier to scale specific components based on demand without affecting the entire system. Cloud platforms and orchestration tools like Kubernetes leverage this capability to optimize resource allocation and performance. For example, scaling a web service might involve running more replicas of a containerized application:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-service
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: web-container
        image: myapp:latest
        ports:
        - containerPort: 80

This flexibility makes it easier to handle traffic spikes while optimizing resources efficiently.

Strengthened security with immutable images

Once developers build container images, they stay unchanged to prevent unauthorized modifications that could introduce vulnerabilities. Security teams can scan images before deployment to ensure that only trusted and verified versions enter production. 

A common practice is scanning images for vulnerabilities before deployment using tools like Trivy:

trivy image myapp:latest

This immutability strengthens cloud security by reducing the risk of tampering and simplifying rollback procedures if an issue arises.

Container images are at the core of cloud-native development, making applications faster to deploy, easier to manage, and more secure. Their role in modern infrastructure continues to grow as organizations embrace scalable, resilient deployment strategies.

Common security risks for container images

While container images streamline deployment, they can also create security risks. Attackers can exploit vulnerabilities as soon as a container runs, so securing images at every stage remains essential.

Be sure to watch out for some of these risks: 

• Vulnerable dependencies: Many container images rely on third-party libraries, which may contain security flaws. Without regular updates, these vulnerabilities create entry points for attackers.

• Misconfigurations: Poorly configured images can expose applications to unauthorized access, data leaks, or privilege escalation attacks. Common misconfigurations include excessive permissions and open network ports.

• Compromised images: If an attacker gains access to a container registry, they can replace trusted images with malicious ones. Additionally, running a compromised image can introduce malware, backdoors, or data exfiltration risks.

• Hardcoded secrets: Storing credentials, API keys, or sensitive data inside an image is a major security flaw. Attackers can access critical systems and exploit them further if they obtain these exposed secrets.

3 best practices for container image management and security

Managing and securing container images is essential for ensuring reliable, efficient, and safe deployments. By implementing these best practices, you can reduce container security risks, optimize performance, and maintain consistency across environments:

1. Keep images updated and secure

Regularly updating container images prevents security vulnerabilities and ensures stability. Outdated images often contain known exploits, which makes them potential attack vectors, but rebuilding images with the latest base versions mitigates these risks.

For example, upgrading from FROM alpine:3.19 to FROM alpine:3.20 inherits security patches from upstream maintainers.

Security scanning also plays a crucial role in maintaining image security. Integrating tools like Trivy, Grype, and Docker Scout into your CI/CD pipeline allows for automatic detection of outdated packages and vulnerabilities before deployment. These tools identify security risks, such as outdated system libraries or exposed credentials.

For instance, in a Node.js project, running these commands updates outdated dependencies, reducing exposure to vulnerabilities:

npm outdated
npm update

Using minimal base images reduces attack surfaces as well. Lightweight images like Alpine Linux contain fewer components, minimizing potential risks compared to full-fledged distributions.

2. Implement strong version control and rollback strategies

Effective version control makes it easier to track image changes, prevent unexpected updates, and roll back when necessary. Be sure to avoid vague tags like latest, as they can lead to unintended updates in production. Instead, use versioned and environment-specific tags:

• Semantic versioning (MAJOR.MINOR.PATCH):

  • Major: Breaking changes (v2.0.0 → v3.0.0)

  • Minor: New features (v1.1.0 → v1.2.0)

  • Patch: Security fixes (v1.2.1 → v1.2.2)

• Branch-based tagging:

  • feature-xyz

  • hotfix-123

  • release-v1.0.0

To tag and push images effectively:

# Build the image  
docker build -t myapp:1.2.3 .  

# Tag for production  
docker tag myapp:1.2.3 mydockerhubuser/myapp:prod  

# Push tags to registry  
docker push mydockerhubuser/myapp:1.2.3  
docker push mydockerhubuser/myapp:prod 

3. Automate image builds, security scanning, and deployments

Using CI/CD pipelines for image builds, security scanning, and deployments improves consistency, minimizes human error, and accelerates delivery. You can use platforms like GitHub Actions, Jenkins, and GitLab CI/CD to streamline rollouts while integrating them with orchestration tools like Kubernetes.

A well-structured Dockerfile also strengthens security by separating build dependencies from runtime components. A multi-stage build helps achieve this:

# First stage: Build the application
FROM node:18 AS builder
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
RUN npm run build

# Second stage: Create a lightweight runtime image
FROM node:18-slim
WORKDIR /app
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/server.js"]

This approach ensures that only necessary files exist in the final image, keeping it lightweight and secure.

To automate image builds and security scanning on code commits, a GitHub Actions workflow could look like this:

name: Build and Push Docker Image
on:
  push:
    branches:
      - main
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v3
      
      - name: Log in to Docker Hub
        uses: docker/login-action@v2
        with:
          username: ${{ secrets.DOCKER_USERNAME }}
          password: ${{ secrets.DOCKER_PASSWORD }}
      
      - name: Build, scan, and push image
        run: |
          docker build -t myapp:latest .
          trivy image --exit-code 1 myapp:latest  # Security scan before push
          docker tag myapp:latest mydockerhubuser/myapp:latest
          docker push mydockerhubuser/myapp:latest

This workflow automatically builds, scans, and deploys images when you push new changes to main, which ensures that only secure, up-to-date images reach production.

Wiz’s approach to container image security

Containers are a core part of modern cloud environments, which makes strong security essential. 

Securing containerized applications requires a proactive strategy—and Wiz simplifies this process with its unified security platform. While Wiz is a cloud-native application protection platform, it also provides cloud security posture management, container and Kubernetes security, vulnerability management, and data protection for complete visibility and protection across AWS, Azure, Google Cloud, and Kubernetes environments.

Not only does Wiz enhance container security, but it also helps teams maintain agility without compromising protection. By integrating security directly into development workflows, Wiz detects vulnerabilities early, scans infrastructure as code for misconfigurations, and manages secrets securely. This approach reduces risk, streamlines compliance, and strengthens security across every layer of your cloud infrastructure.

Ready to secure your container images? Get Wiz’s free cheat sheet with expert best practices today.

What's running in your containers?

Learn why CISOs at the fastest growing companies use Wiz to uncover blind spots in their containerized environments.

Get a demo