1. Introduction

This tutorial will guide you on how to scan Docker images for vulnerabilities using the open-source tool, Trivy. Image scanning is an important process that helps in identifying and fixing known vulnerabilities in Docker images which can prevent potential security threats.

By the end of this tutorial, you will learn:
- The importance of scanning Docker images for vulnerabilities.
- How to install Trivy.
- How to use Trivy to scan Docker images.

Prerequisites:
- Basic understanding of Docker and Docker images.
- Docker installed on your machine.

2. Step-by-Step Guide

2.1 Installing Trivy

Trivy is a simple and comprehensive vulnerability scanner for containers, which is suitable for CI/CD environments. Install Trivy using the following command:

$ curl -sfL https://raw.githubusercontent.com/aquasecurity/trivy/main/contrib/install.sh | sh -s -- -b /usr/local/bin

2.2 Scanning Docker Images using Trivy

Once Trivy is installed, you can scan Docker images for vulnerabilities with the following command:

$ trivy image [options] image_name

For example, to scan the alpine:3.10.2 Docker image, you would use:

$ trivy image alpine:3.10.2

3. Code Examples

3.1 Scanning a Docker image with Trivy

# Pull the Docker image you want to scan
$ docker pull node:14

# Scan the Docker image using Trivy
$ trivy image node:14

The output will show a list of possible vulnerabilities found in the Docker image, together with their severity levels (CRITICAL, HIGH, MEDIUM, LOW, UNKNOWN).

4. Summary

In this tutorial, we have learned the importance of scanning Docker images for vulnerabilities, how to install Trivy, and how to use Trivy to scan Docker images.

To further your learning, you could explore:
- How to automate the scanning process in a CI/CD pipeline.
- Other tools for scanning Docker images, like Clair, Dockle, etc.

Additional resources:
- Trivy GitHub repository
- Docker documentation

5. Practice Exercises

1. Exercise 1: Download and scan the nginx:latest Docker image using Trivy. What are the critical vulnerabilities found?
2. Exercise 2: Download the ubuntu:18.04 Docker image and scan it using Trivy. Compare the vulnerabilities found with those from the nginx:latest Docker image.

Solutions and explanations:

1. Use the commands: docker pull nginx:latest and trivy image nginx:latest. The output will list the vulnerabilities found, if any.

2. Use the commands: docker pull ubuntu:18.04 and trivy image ubuntu:18.04. The output lists the vulnerabilities found, if any. Compare this output with the previous one to see the differences in vulnerabilities between the two Docker images.

Tips for further practice:

Experiment with different Docker images and observe the differences in the vulnerabilities found. Try to understand why these differences exist and how they can be mitigated.