comment on using init() for libcontainer handling

drivers/shared/executor/libcontainer_nsenter_linux.go

@@ -12,6 +12,9 @@ import (
 // init is only run on linux and is used when the LibcontainerExecutor starts
 // a new process. The libcontainer shim takes over the process, setting up the
 // configured isolation and limitions before execve into the user process
+//
+// This subcommand handler is implemented as an `init`, libcontainer shim is handled anywhere
+// this package is used (including tests) without needing to write special command handler.
 func init() {
 	if len(os.Args) > 1 && os.Args[1] == "libcontainer-shim" {
 		runtime.GOMAXPROCS(1)