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

README.md

@@ -144,10 +144,23 @@ The tradeoff is that musl has slightly different behavior than glibc in some edg
 
 ## Configuration
 
-krust looks for configuration at `~/.config/krust/config.toml`:
+### Project Configuration (Cargo.toml)
+
+You can configure krust on a per-project basis by adding a `[package.metadata.krust]` section to your project's `Cargo.toml`:
 
 ```toml
-base_image = "gcr.io/distroless/static:nonroot"
+[package.metadata.krust]
+base-image = "cgr.dev/chainguard/static:latest"  # Override the default base image
+```
+
+This is the idiomatic way to configure build tools in Rust, similar to how `cargo-deb`, `wasm-pack`, and other Cargo extensions work.
+
+### Global Configuration
+
+krust also looks for global configuration at `~/.config/krust/config.toml`:
+
+```toml
+base_image = "cgr.dev/chainguard/static:latest"  # Default base image for all projects
 default_registry = "ghcr.io"
 
 [build]
@@ -160,6 +173,13 @@ password = "mytoken"
 
 Note: Registry authentication is not yet implemented. Currently, krust uses anonymous authentication.
 
+### Configuration Precedence
+
+When determining the base image, krust uses this precedence order:
+1. Project-specific config in `Cargo.toml` (highest priority)
+2. Global config in `~/.config/krust/config.toml`
+3. Built-in default: `cgr.dev/chainguard/static:latest` (lowest priority)
+
 ## Key Features
 
 - **Docker-free** - Builds OCI container images without requiring Docker daemon

example/hello-krust/Cargo.toml

@@ -4,3 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+
+# Example: Configure krust to use a specific base image for this project
+[package.metadata.krust]
+base-image = "cgr.dev/chainguard/static:latest"

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());
     }

src/main.rs

@@ -38,6 +38,14 @@ async fn main() -> Result<()> {
             let config = Config::load()?;
             let project_path = path.unwrap_or_else(|| PathBuf::from("."));
 
+            // Load project-specific config from Cargo.toml
+            let project_config = Config::load_project_config(&project_path)?;
+
+            // Determine base image (project config takes precedence)
+            let base_image = project_config
+                .base_image
+                .unwrap_or(config.base_image.clone());
+
             // Determine the image name
             let image_ref = if let Some(image) = image {
                 // Use explicit image if provided
@@ -56,8 +64,7 @@ async fn main() -> Result<()> {
             let binary_path = builder.build()?;
 
             // Build container image
-            let image_builder =
+            let image_builder = ImageBuilder::new(binary_path, base_image, platform.clone());
-                ImageBuilder::new(binary_path, config.base_image.clone(), platform.clone());
 
             let (config_data, layer_data, manifest) = image_builder.build()?;
 

tests/integration_test.rs

@@ -53,6 +53,7 @@ fn test_build_requires_repo_or_image() -> Result<()> {
     let mut cmd = Command::cargo_bin("krust")?;
     cmd.arg("build")
         .arg("--no-push")
+        .arg(".") // Explicitly pass current directory
         .current_dir(&example_dir)
         .env_remove("KRUST_REPO");
 
@@ -71,6 +72,7 @@ fn test_build_with_krust_repo_env() -> Result<()> {
     let mut cmd = Command::cargo_bin("krust")?;
     cmd.arg("build")
         .arg("--no-push")
+        .arg(".") // Explicitly pass current directory
         .env("KRUST_REPO", "test.local")
         .current_dir(&example_dir);
 
@@ -95,6 +97,7 @@ fn test_command_substitution_syntax() -> Result<()> {
         .arg("--no-push")
         .arg("--image")
         .arg("test.local/hello:latest")
+        .arg(".") // Explicitly pass current directory
         .current_dir(&example_dir)
         .output()?;
 
@@ -122,6 +125,7 @@ fn test_verbose_logging() -> Result<()> {
         .arg("--no-push")
         .arg("--image")
         .arg("test.local/hello:latest")
+        .arg(".") // Explicitly pass current directory
         .current_dir(&example_dir);
 
     cmd.assert()
@@ -151,6 +155,7 @@ fn test_full_build_and_run_workflow() -> Result<()> {
     let mut cmd = Command::cargo_bin("krust")?;
     let output = cmd
         .arg("build")
+        .arg(".") // Explicitly pass current directory
         .env("KRUST_REPO", "ttl.sh/krust-test")
         .current_dir(&example_dir)
         .output()?;