docs: document docker_volumes config for shared host directories

The Docker backend already supports user-configured volume mounts via
docker_volumes, but it was undocumented — missing from DEFAULT_CONFIG,
cli.py defaults, and configuration docs.

Changes:
- hermes_cli/config.py: Add docker_volumes to DEFAULT_CONFIG with
  inline documentation and examples
- cli.py: Add docker_volumes to load_cli_config defaults
- configuration.md: Full Docker Volume Mounts section with YAML
  examples, use cases (providing files, receiving outputs, shared
  workspaces), and env var alternative

hermes_cli/config.py

@@ -77,6 +77,10 @@ DEFAULT_CONFIG = {
         "container_memory": 5120,       # MB (default 5GB)
         "container_disk": 51200,        # MB (default 50GB)
         "container_persistent": True,   # Persist filesystem across sessions
+        # Docker volume mounts — share host directories with the container.
+        # Each entry is "host_path:container_path" (standard Docker -v syntax).
+        # Example: ["/home/user/projects:/workspace/projects", "/data:/data"]
+        "docker_volumes": [],
     },
     
     "browser": {