Add Docker container support for Linux

## Problem The current containerization implementation (Phase 1 & 2) uses macOS-specific `container` CLI tool via `AppleContainerRuntime`. This doesn't work on Linux systems. ## Requirements - Support Docker containers on Linux - Provide same functionality as AppleContainerRuntime - Use existing ContainerRuntime interface for consistency ## Implementation Approach Create a `DockerContainerRuntime` that: - Implements the `ContainerRuntime` interface - Uses Docker CLI or Docker SDK for Node.js - Provides equivalent isolation and mounting capabilities - Works with existing `WorkspaceContainerManager` ## Technical Details The ContainerRuntime interface already exists: ```typescript interface ContainerRuntime { create(config: ContainerConfig): string | Promise<string>; start(containerId: string): Promise<void>; stop(containerId: string, timeout?: number): Promise<void>; remove(containerId: string): Promise<void>; exec(containerId: string, options: ExecOptions): Promise<ExecResult>; inspect(containerId: string): ContainerInfo | Promise<ContainerInfo>; list(): ContainerInfo[] | Promise<ContainerInfo[]>; translateToContainer(hostPath: string, containerId: string): string; translateToHost(containerPath: string, containerId: string): string; } ``` ## Platform Detection - Runtime selection should be automatic based on platform - Could use factory pattern: `ContainerRuntimeFactory.create()` - macOS → AppleContainerRuntime - Linux → DockerContainerRuntime - Windows → Could use WSL2 + Docker ## Dependencies - Docker daemon must be running - Consider graceful degradation if Docker not available - May need `dockerode` npm package or use Docker CLI ## Related - Phase 1: Container Runtime (currently macOS only) - Cross-platform support - Container abstraction layer ## Acceptance Criteria - [ ] DockerContainerRuntime implements ContainerRuntime interface - [ ] Works on Linux with Docker installed - [ ] All existing tests pass with Docker backend - [ ] Automatic runtime selection based on platform - [ ] Documentation for Docker setup requirements - [ ] Tests run on Linux CI