feat: Add multi-arch support

- Support building for multiple platforms with --platform flag
- Accept comma-separated platforms or multiple --platform flags
- Default to building for linux/amd64 and linux/arm64
- Add ARM runner (ubuntu-24.04-arm) to CI matrix
- Create manifest list for multi-arch images (TODO: push to registry)
- Update integration tests to handle multi-platform builds
- Add cross-compilation setup instructions to README
- Add .cargo/config.toml for local development

Breaking changes:
- Default behavior now builds for multiple platforms (amd64+arm64)
- Use --platform linux/amd64 to build for single platform

src/cli/mod.rs

@@ -25,9 +25,10 @@ pub enum Commands {
         #[arg(short, long, env = "KRUST_IMAGE")]
         image: Option<String>,
 
-        /// Target platform (e.g., linux/amd64, linux/arm64)
-        #[arg(long, default_value = "linux/amd64")]
-        platform: String,
+        /// Target platforms (e.g., linux/amd64, linux/arm64)
+        /// Can be specified multiple times or as a comma-separated list
+        #[arg(long, value_delimiter = ',')]
+        platform: Option<Vec<String>>,
 
         /// Skip pushing the image to the registry after building
         #[arg(long)]