Add basic spec and mounts for Darwin

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

oci/spec.go

@@ -21,11 +21,11 @@ import (
 	"path/filepath"
 	"runtime"
 
-	"github.com/containerd/containerd/namespaces"
-	"github.com/containerd/containerd/platforms"
+	"github.com/opencontainers/runtime-spec/specs-go"
 
 	"github.com/containerd/containerd/containers"
-	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/containerd/containerd/namespaces"
+	"github.com/containerd/containerd/platforms"
 )
 
 const (
@@ -66,15 +66,19 @@ func generateDefaultSpecWithPlatform(ctx context.Context, platform, id string, s
 		return err
 	}
 
-	if plat.OS == "windows" {
+	switch plat.OS {
+	case "windows":
 		err = populateDefaultWindowsSpec(ctx, s, id)
-	} else {
+	case "darwin":
+		err = populateDefaultDarwinSpec(s)
+	default:
 		err = populateDefaultUnixSpec(ctx, s, id)
 		if err == nil && runtime.GOOS == "windows" {
 			// To run LCOW we have a Linux and Windows section. Add an empty one now.
 			s.Windows = &specs.Windows{}
 		}
 	}
+
 	return err
 }
 
@@ -207,3 +211,12 @@ func populateDefaultWindowsSpec(ctx context.Context, s *Spec, id string) error {
 	}
 	return nil
 }
+
+func populateDefaultDarwinSpec(s *Spec) error {
+	*s = Spec{
+		Version: specs.Version,
+		Root:    &specs.Root{},
+		Process: &specs.Process{Cwd: "/"},
+	}
+	return nil
+}