ctr: make kill use stop-signal by default

The OCI image specification includes a `StopSignal` field in the image
configuration, denoting the system call signal to be sent to the
container to exit.  This commit adds a new `WithImageStopSignal` container
option that can be used for storing the `StopSignal` field as a label on
the container.  This commit also adjusts `ctr run` to call
`WithImageStopSignal` and `ctr tasks kill` to send the signal stored in
that label by default.

Signed-off-by: Samuel Karp <skarp@amazon.com>

cmd/ctr/commands/tasks/kill.go

@@ -23,6 +23,8 @@ import (
 	"github.com/urfave/cli"
 )
 
+const defaultSignal = "SIGTERM"
+
 var killCommand = cli.Command{
 	Name:      "kill",
 	Usage:     "signal a container (default: SIGTERM)",
@@ -30,7 +32,7 @@ var killCommand = cli.Command{
 	Flags: []cli.Flag{
 		cli.StringFlag{
 			Name:  "signal, s",
-			Value: "SIGTERM",
+			Value: "",
 			Usage: "signal to send to the container",
 		},
 		cli.StringFlag{
@@ -47,7 +49,7 @@ var killCommand = cli.Command{
 		if id == "" {
 			return errors.New("container id must be provided")
 		}
-		signal, err := containerd.ParseSignal(context.String("signal"))
+		signal, err := containerd.ParseSignal(defaultSignal)
 		if err != nil {
 			return err
 		}
@@ -74,6 +76,17 @@ var killCommand = cli.Command{
 		if err != nil {
 			return err
 		}
+		if context.String("signal") != "" {
+			signal, err = containerd.ParseSignal(context.String("signal"))
+			if err != nil {
+				return err
+			}
+		} else {
+			signal, err = containerd.GetStopSignal(ctx, container, signal)
+			if err != nil {
+				return err
+			}
+		}
 		task, err := container.Task(ctx, nil)
 		if err != nil {
 			return err