make exit code of exec/container consist with docker

The exit code from `docker run` gives information about why the container
failed to run or why it exited.  When `docker run` exits with a non-zero code,
the exit codes follow the `chroot` standard, see below:

**_125_** if the error is with Docker daemon **_itself_**

    $ docker run --foo busybox; echo $?
    # flag provided but not defined: --foo
      See 'docker run --help'.
      125

**_126_** if the **_contained command_** cannot be invoked

    $ docker run busybox /etc; echo $?
    # docker: Error response from daemon: Container command '/etc' could not be invoked.
      126

**_127_** if the **_contained command_** cannot be found

    $ docker run busybox foo; echo $?
    # docker: Error response from daemon: Container command 'foo' not found or does not exist.
      127

**_Exit code_** of **_contained command_** otherwise

    $ docker run busybox /bin/sh -c 'exit 3'; echo $?
    # 3

Signed-off-by: Gao feng <omarapazanadi@gmail.com>

Author: gao-feng

src/container.c

@@ -624,8 +624,15 @@ static int hyper_container_init(void *data)
 	execvp(container->exec.argv[0], container->exec.argv);
 	perror("exec container command failed");
 
+	 /* the exit codes follow the `chroot` standard,
+	    see docker/docs/reference/run.md#exit-status */
+	if (errno == ENOENT)
+		_exit(127);
+	else if (errno == EACCES)
+		_exit(126);
+
 fail:
-	_exit(-1);
+	_exit(125);
 }
 
 static int hyper_setup_pty(struct hyper_container *c)

src/exec.c

@@ -590,10 +590,20 @@ static int hyper_do_exec_cmd(void *data)
 		goto exit;
 	}
 
-	if (execvp(exec->argv[0], exec->argv) < 0)
+	if (execvp(exec->argv[0], exec->argv) < 0) {
 		perror("exec failed");
+
+		 /* the exit codes follow the `chroot` standard,
+		    see docker/docs/reference/run.md#exit-status */
+		if (errno == ENOENT)
+			exit(127);
+		else if (errno == EACCES)
+			exit(126);
+
+	}
+
 exit:
-	_exit(ret);
+	_exit(125);
 out:
 	hyper_send_type(arg->pipe[1], ret ? ERROR : READY);
 	_exit(ret);