Launch sandboxed containers from task service

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

containers/containers.go

@@ -78,6 +78,8 @@ type Container struct {
 	Extensions map[string]typeurl.Any
 
 	// SandboxID is an identifier of sandbox this container belongs to.
+	//
+	// This property is optional, but can't be changed after creation.
 	SandboxID string
 }