Server doesn't validate Docker image existence before creating containers

[alanops]

Problem

The server attempts to create containers from Docker images without checking if those images exist, leading to confusing errors.

Current Behavior

• Server maps scenario IDs to image names
• Directly attempts docker run without validation
• If image doesn't exist: "Unable to find image 'devopslearn/scenario-keycloak-crashloop:latest' locally"

Expected Behavior

• Server should check if required images are built
• Provide clear error messages if images are missing
• Guide users to run make scenario-build

Root Cause

In src/server/index.ts, the startScenarioContainer function:

const imageMap: Record<string, string> = {
  'k8s-crashloop': 'devopslearn/scenario-keycloak-crashloop',
  // ...
};
// No validation before:
const child = spawn('docker', args);

Solution

1. Add image existence check before container creation
2. Return meaningful error to client if image missing
3. Include instructions for building images

Impact

• Poor user experience when images aren't built
• Cryptic Docker errors instead of actionable messages
• Users don't know they need to run make scenario-build