feat: Add automatic container image security scanning using Trivy

[haroon0x]

What this PR does / why we need it:

Implements automatic container image vulnerability scanning using Trivy in the CI/CD pipeline. Every image built by the project will now be scanned for security vulnerabilities before being pushed to the registry.

This addresses the need for regular, automated security scanning rather than relying on manual scans before releases.
Fixes #2095

Changes

Modified: .github/workflows/template-publish-image/action.yaml

• Added scan-image input parameter (default: true) to enable/disable scanning
• Added "Build Image for Scanning" step - builds single-arch image locally for Trivy
• Added "Run Trivy Vulnerability Scanner" step - scans for CRITICAL and HIGH severity vulnerabilities
• Added "Upload Trivy Scan Results" step - uploads SARIF results to GitHub Security tab

How it works

Build (amd64) → Trivy Scan → Upload SARIF → Multi-arch Build → Push

Configuration

Setting	Value	Reason
Severity	CRITICAL,HIGH	Focus on actionable vulnerabilities
ignore-unfixed	true	Skip vulnerabilities without available fixes
Format	sarif	GitHub Code Scanning integration
Scan arch	linux/amd64	Speed optimization (vulns are consistent across architectures)

Note: This implementation is non-blocking by default - scan results are uploaded for visibility but do not fail the build. This can be changed to a blocking gate by adding exit-code: '1' to the Trivy action configuration.

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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