Add Cargo.toml-based configuration for base image

- Add support for [package.metadata.krust] in Cargo.toml
- Change default base image to cgr.dev/chainguard/static:latest
- Add ProjectConfig struct to load project-specific settings
- Update documentation and example to show configuration usage
- Fix integration tests to explicitly pass directory argument
- This is the idiomatic way for Rust build tools to be configured

src/config/mod.rs

@@ -1,6 +1,6 @@
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 #[cfg(test)]
 mod tests;
@@ -45,7 +45,7 @@ pub struct RegistryAuth {
 }
 
 fn default_base_image() -> String {
-    "gcr.io/distroless/static:nonroot".to_string()
+    "cgr.dev/chainguard/static:latest".to_string()
 }
 
 impl Default for Config {
@@ -59,6 +59,14 @@ impl Default for Config {
     }
 }
 
+/// Project-specific configuration from Cargo.toml
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+pub struct ProjectConfig {
+    /// Base image for this project
+    #[serde(rename = "base-image")]
+    pub base_image: Option<String>,
+}
+
 impl Config {
     pub fn load() -> anyhow::Result<Self> {
         if let Some(config_dir) = dirs::config_dir() {
@@ -71,4 +79,27 @@ impl Config {
         }
         Ok(Config::default())
     }
+
+    /// Load project-specific configuration from Cargo.toml
+    pub fn load_project_config(project_path: &Path) -> anyhow::Result<ProjectConfig> {
+        let cargo_toml_path = project_path.join("Cargo.toml");
+        if !cargo_toml_path.exists() {
+            return Ok(ProjectConfig::default());
+        }
+
+        let content = std::fs::read_to_string(&cargo_toml_path)?;
+        let value: toml::Value = toml::from_str(&content)?;
+
+        // Look for [package.metadata.krust] section
+        if let Some(metadata) = value
+            .get("package")
+            .and_then(|p| p.get("metadata"))
+            .and_then(|m| m.get("krust"))
+        {
+            let project_config: ProjectConfig = metadata.clone().try_into()?;
+            return Ok(project_config);
+        }
+
+        Ok(ProjectConfig::default())
+    }
 }

src/config/tests.rs

@@ -5,7 +5,7 @@ mod tests {
     #[test]
     fn test_default_config() {
         let config = Config::default();
-        assert_eq!(config.base_image, "gcr.io/distroless/static:nonroot");
+        assert_eq!(config.base_image, "cgr.dev/chainguard/static:latest");
         assert!(config.default_registry.is_none());
         assert!(config.registries.is_empty());
     }