feat: Implement Docker runtime for MCP servers

[initcron]

Summary

Implement Docker runtime support to spawn MCP servers as Docker containers, enabling better isolation and reproducibility.

Parent Epic

Part of #1 - Production Kubernetes & Container Support

Implementation Details

RuntimeConfig Addition

pub enum RuntimeConfig {
    LocalProcess { ... },
    RemoteSse { ... },
    Docker {
        image: String,
        command: Option<Vec<String>>,
        env: HashMap<String, String>,
        volumes: Vec<VolumeMount>,
        network: Option<String>,
        working_dir: Option<String>,
    },
}

Features Required

• Container lifecycle management (create, start, stop, remove)
• Volume mounting support for data persistence
• Environment variable injection
• Image pull handling (with authentication support)
• Container networking configuration
• Resource limits (CPU, memory)
• Health check configuration

Example Catalog Entry

servers:
  - id: postgres-mcp
    runtime:
      type: docker
      image: mcp/postgres:latest
      env:
        DATABASE_URL: "${DATABASE_URL}"
      volumes:
        - /data:/app/data:ro

Acceptance Criteria

• Can define server with type: docker in catalog.yaml
• Gateway spawns container on first tool call
• Container is stopped after idle timeout
• Volumes are properly mounted
• Environment variables are injected
• Logs are captured and available via API

Dependencies

• Docker Engine or compatible runtime
• bollard crate for Docker API

References

• bollard - Rust Docker client
• Docker Engine API