feat: add image command invoking container engine

[PrasanthYT]

Overview

This change introduces the initial container scanning engine structure and integrates it with the existing engine abstraction layer.

The goal is to establish the container scanning entry point without implementing scanning logic yet.

New scanr-container Crate

Added a new workspace crate:

crates/scanr-container

Dependencies:

• scanr-engine
• scanr-sca

This crate will host container-specific scanning logic.

Container Engine Structure

Implemented a new engine:

pub struct ContainerEngine {
    sca_engine: ScaEngine,
}

The engine implements the unified engine contract:

impl ScanEngine for ContainerEngine

At this stage it returns a placeholder ScanResult.

CLI Integration

Added new command:

scanr image <target>

Execution flow:

CLI
  → ContainerEngine
      → placeholder ScanResult

This establishes the container scanning entry point.

Behavior

• Command compiles and executes
• Placeholder result returned
• No container scanning logic implemented yet
• No changes to existing SCA functionality

Impact

This change establishes the structural foundation required for future container scanning features.

It prepares Scanr for multi-engine scanning support while keeping current functionality unchanged.