# [Docs] Document WithContainerFiles API

## Summary

Aspire 9.4 introduced `WithContainerFiles` for copying files into containers with permission control. Not documented outside what's-new.

## What's Missing

### Core API
```csharp
var myContainer = builder.AddContainer("myapp", "myapp:latest")
    // Simple file copying
    .WithContainerFiles("/app/config", "./config-files")
    
    // With ownership
    .WithContainerFiles("/app/data", "./data", 
        defaultOwner: 1000, 
        defaultGroup: 1000)
    
    // With permissions
    .WithContainerFiles("/app/scripts", "./scripts", 
        umask: UnixFileMode.UserRead | UnixFileMode.UserWrite);
```

### Dynamic File Generation
```csharp
var dynamicContainer = builder.AddContainer("worker", "worker:latest")
    .WithContainerFiles("/app/runtime-config", async (context, ct) => {
        var configFile = new ContainerFileSystemItem {
            Name = "app.json",
            Contents = JsonSerializer.SerializeToUtf8Bytes(new { 
                Environment = "Production" 
            })
        };
        return new[] { configFile };
    });
```

## Suggested Content

1. **Static file mounting** - copying directories into containers
2. **File permissions** - owner, group, umask
3. **Dynamic file generation** - callback-based content creation
4. **Use cases** - config files, certificates, init scripts
5. **Examples** - nginx config, app settings, SSL certs

## Why It Matters

Configuration file mounting is a common need. Without docs, users resort to complex volume mounts.

## Labels

`documentation`, `area-docs`, `containers`