Implement image pull retry for tink-worker image

There could be races where linuxkit network or dns may not have been
fully set up and functional yet and image pull fails because of that.

Signed-off-by: Pooja Trivedi <tripooja@amazon.com>

Author: ptrivedi

bootkit/main.go

@@ -47,6 +47,9 @@ type tinkConfig struct {
 	tinkServerTLS string
 }
 
+const imagePullRetryAttempts = 10
+const retrySleepSeconds = 5
+
 func main() {
 	fmt.Println("Starting BootKit")
 
@@ -146,11 +149,28 @@ func main() {
 
 	fmt.Printf("Pulling image [%s]", imageName)
 
-	out, err := cli.ImagePull(ctx, imageName, pullOpts)
-	if err != nil {
-		panic(err)
+	// TODO: Ideally if this function becomes a loop that runs forever and keeps retrying 
+	// anything that failed, this retry would not be needed. For now, this addresses the specific
+	// race condition case of when the linuxkit network or dns is in the process of, but not quite
+	// fully set up yet.
+
+	failedImagePull := true
+
+	var out io.ReadCloser
+	for i := 0; i < imagePullRetryAttempts; i++ {
+		out, err = cli.ImagePull(ctx, imageName, pullOpts)
+		if err == nil {
+			failedImagePull = false
+			break
+		}
+		fmt.Printf("Error pulling image [%s] [%v]. Retrying after %d seconds...\n", imageName, err, retrySleepSeconds)
+		time.Sleep(time.Second * retrySleepSeconds)
 	}
 
+	if failedImagePull {
+		panic(err)
+ 	}
+
 	_, err = io.Copy(os.Stdout, out)
 	if err != nil {
 		panic(err)