feat(wfe-buildkit): rewrite to use buildkit-client gRPC instead of CLI

Replaced buildctl CLI shell-out with direct gRPC communication via
buildkit-client crate. Connects to buildkitd daemon over Unix socket
or TCP with optional TLS.

Implementation:
- connect() with custom tonic UnixStream connector
- execute_build() implementing the solve protocol directly against
  ControlClient (session setup, file sync, frontend attributes)
- Extracts digest from containerimage.digest in solve response

Added custom lima template (test/lima/wfe-test.yaml) that provides
both buildkitd and containerd with host-forwarded Unix sockets for
reproducible integration testing.

E2E tests against real buildkitd daemon via WFE_BUILDKIT_ADDR env var.
54 tests total. 89% line coverage (cargo-llvm-cov with E2E).

wfe-buildkit/tests/integration_test.rs

@@ -0,0 +1,231 @@
+//! Integration tests for wfe-buildkit using a real BuildKit daemon.
+//!
+//! These tests require a running BuildKit daemon. The socket path is read
+//! from `WFE_BUILDKIT_ADDR`, falling back to
+//! `unix:///Users/sienna/.lima/wfe-test/sock/buildkitd.sock`.
+//!
+//! If the daemon is not available, the tests are skipped gracefully.
+
+use std::collections::HashMap;
+use std::path::Path;
+
+use wfe_buildkit::config::{BuildkitConfig, TlsConfig};
+use wfe_buildkit::BuildkitStep;
+
+use wfe_core::models::{ExecutionPointer, WorkflowInstance, WorkflowStep};
+use wfe_core::traits::step::{StepBody, StepExecutionContext};
+
+/// Get the BuildKit daemon address from the environment or use the default.
+fn buildkit_addr() -> String {
+    std::env::var("WFE_BUILDKIT_ADDR").unwrap_or_else(|_| {
+        "unix:///Users/sienna/.lima/wfe-test/sock/buildkitd.sock".to_string()
+    })
+}
+
+/// Check whether the BuildKit daemon socket is reachable.
+fn buildkitd_available() -> bool {
+    let addr = buildkit_addr();
+    if let Some(path) = addr.strip_prefix("unix://") {
+        Path::new(path).exists()
+    } else {
+        // For TCP endpoints, optimistically assume available.
+        true
+    }
+}
+
+fn make_test_context(
+    step_name: &str,
+) -> (
+    WorkflowStep,
+    ExecutionPointer,
+    WorkflowInstance,
+) {
+    let mut step = WorkflowStep::new(0, "buildkit");
+    step.name = Some(step_name.to_string());
+    let pointer = ExecutionPointer::new(0);
+    let instance = WorkflowInstance::new("test-wf", 1, serde_json::json!({}));
+    (step, pointer, instance)
+}
+
+#[tokio::test]
+async fn build_simple_dockerfile_via_grpc() {
+    if !buildkitd_available() {
+        eprintln!(
+            "SKIP: BuildKit daemon not available at {}",
+            buildkit_addr()
+        );
+        return;
+    }
+
+    // Create a temp directory with a trivial Dockerfile.
+    let tmp = tempfile::tempdir().unwrap();
+    let dockerfile = tmp.path().join("Dockerfile");
+    std::fs::write(
+        &dockerfile,
+        "FROM alpine:latest\nRUN echo built\n",
+    )
+    .unwrap();
+
+    let config = BuildkitConfig {
+        dockerfile: "Dockerfile".to_string(),
+        context: tmp.path().to_string_lossy().to_string(),
+        target: None,
+        tags: vec![],
+        build_args: HashMap::new(),
+        cache_from: vec![],
+        cache_to: vec![],
+        push: false,
+        output_type: None,
+        buildkit_addr: buildkit_addr(),
+        tls: TlsConfig::default(),
+        registry_auth: HashMap::new(),
+        timeout_ms: Some(120_000), // 2 minutes
+    };
+
+    let mut step = BuildkitStep::new(config);
+
+    let (ws, pointer, instance) = make_test_context("integration-build");
+    let cancel = tokio_util::sync::CancellationToken::new();
+    let ctx = StepExecutionContext {
+        item: None,
+        execution_pointer: &pointer,
+        persistence_data: None,
+        step: &ws,
+        workflow: &instance,
+        cancellation_token: cancel,
+    };
+
+    let result = step.run(&ctx).await.expect("build should succeed");
+
+    assert!(result.proceed);
+
+    let data = result.output_data.expect("should have output_data");
+    let obj = data.as_object().expect("output_data should be an object");
+
+    // Without tags/push, BuildKit does not produce a digest in the exporter
+    // response. The build succeeds but the digest is absent.
+    assert!(
+        obj.contains_key("integration-build.stdout"),
+        "expected stdout key, got: {:?}",
+        obj.keys().collect::<Vec<_>>()
+    );
+    assert!(
+        obj.contains_key("integration-build.stderr"),
+        "expected stderr key, got: {:?}",
+        obj.keys().collect::<Vec<_>>()
+    );
+
+    // If a digest IS present (e.g., newer buildkitd versions), validate its format.
+    if let Some(digest_val) = obj.get("integration-build.digest") {
+        let digest = digest_val.as_str().unwrap();
+        assert!(
+            digest.starts_with("sha256:"),
+            "digest should start with sha256:, got: {digest}"
+        );
+        assert_eq!(
+            digest.len(),
+            7 + 64,
+            "digest should be sha256:<64hex>, got: {digest}"
+        );
+    }
+}
+
+#[tokio::test]
+async fn build_with_build_args() {
+    if !buildkitd_available() {
+        eprintln!(
+            "SKIP: BuildKit daemon not available at {}",
+            buildkit_addr()
+        );
+        return;
+    }
+
+    let tmp = tempfile::tempdir().unwrap();
+    let dockerfile = tmp.path().join("Dockerfile");
+    std::fs::write(
+        &dockerfile,
+        "FROM alpine:latest\nARG MY_VAR=default\nRUN echo \"value=$MY_VAR\"\n",
+    )
+    .unwrap();
+
+    let mut build_args = HashMap::new();
+    build_args.insert("MY_VAR".to_string(), "custom_value".to_string());
+
+    let config = BuildkitConfig {
+        dockerfile: "Dockerfile".to_string(),
+        context: tmp.path().to_string_lossy().to_string(),
+        target: None,
+        tags: vec![],
+        build_args,
+        cache_from: vec![],
+        cache_to: vec![],
+        push: false,
+        output_type: None,
+        buildkit_addr: buildkit_addr(),
+        tls: TlsConfig::default(),
+        registry_auth: HashMap::new(),
+        timeout_ms: Some(120_000),
+    };
+
+    let mut step = BuildkitStep::new(config);
+
+    let (ws, pointer, instance) = make_test_context("build-args-test");
+    let cancel = tokio_util::sync::CancellationToken::new();
+    let ctx = StepExecutionContext {
+        item: None,
+        execution_pointer: &pointer,
+        persistence_data: None,
+        step: &ws,
+        workflow: &instance,
+        cancellation_token: cancel,
+    };
+
+    let result = step.run(&ctx).await.expect("build with args should succeed");
+    assert!(result.proceed);
+
+    let data = result.output_data.expect("should have output_data");
+    let obj = data.as_object().unwrap();
+
+    // Build should complete and produce output data entries.
+    assert!(
+        obj.contains_key("build-args-test.stdout"),
+        "expected stdout key, got: {:?}",
+        obj.keys().collect::<Vec<_>>()
+    );
+}
+
+#[tokio::test]
+async fn connect_to_unavailable_daemon_returns_error() {
+    // Use a deliberately wrong address to test error handling.
+    let config = BuildkitConfig {
+        dockerfile: "Dockerfile".to_string(),
+        context: ".".to_string(),
+        target: None,
+        tags: vec![],
+        build_args: HashMap::new(),
+        cache_from: vec![],
+        cache_to: vec![],
+        push: false,
+        output_type: None,
+        buildkit_addr: "unix:///tmp/nonexistent-buildkitd.sock".to_string(),
+        tls: TlsConfig::default(),
+        registry_auth: HashMap::new(),
+        timeout_ms: Some(5_000),
+    };
+
+    let mut step = BuildkitStep::new(config);
+
+    let (ws, pointer, instance) = make_test_context("error-test");
+    let cancel = tokio_util::sync::CancellationToken::new();
+    let ctx = StepExecutionContext {
+        item: None,
+        execution_pointer: &pointer,
+        persistence_data: None,
+        step: &ws,
+        workflow: &instance,
+        cancellation_token: cancel,
+    };
+
+    let err = step.run(&ctx).await;
+    assert!(err.is_err(), "should fail when daemon is unavailable");
+}