Add oci-hook command to containerd

This allows many different commands to be used as OCI hooks.  It allows
these commands to template out different args and env vars so that
normal commands can accept the OCI spec State payload over stdin.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container_test.go

@@ -22,6 +22,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
 	"syscall"
@@ -34,6 +35,7 @@ import (
 	"github.com/containerd/containerd/oci"
 	_ "github.com/containerd/containerd/runtime"
 	"github.com/containerd/typeurl"
+	specs "github.com/opencontainers/runtime-spec/specs-go"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/windows/hcsshimtypes"
@@ -1469,3 +1471,61 @@ func TestContainerLabels(t *testing.T) {
 		t.Fatalf("expected label \"test\" to be \"no\"")
 	}
 }
+
+func TestContainerHook(t *testing.T) {
+	t.Parallel()
+
+	client, err := newClient(t, address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	var (
+		image       Image
+		ctx, cancel = testContext()
+		id          = t.Name()
+	)
+	defer cancel()
+
+	image, err = client.GetImage(ctx, testImage)
+	if err != nil {
+		t.Fatal(err)
+	}
+	hook := func(_ context.Context, _ oci.Client, _ *containers.Container, s *specs.Spec) error {
+		if s.Hooks == nil {
+			s.Hooks = &specs.Hooks{}
+		}
+		path, err := exec.LookPath("containerd")
+		if err != nil {
+			return err
+		}
+		psPath, err := exec.LookPath("ps")
+		if err != nil {
+			return err
+		}
+		s.Hooks.Prestart = []specs.Hook{
+			{
+				Path: path,
+				Args: []string{
+					"containerd",
+					"oci-hook", "--",
+					psPath, "--pid", "{{pid}}",
+				},
+				Env: os.Environ(),
+			},
+		}
+		return nil
+	}
+	container, err := client.NewContainer(ctx, id, WithNewSpec(oci.WithImageConfig(image), hook), WithNewSnapshot(id, image))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer container.Delete(ctx, WithSnapshotCleanup)
+
+	task, err := container.NewTask(ctx, empty())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer task.Delete(ctx, WithProcessKill)
+}