如何在CSS网格项目周围设置内部边框？

<style>
  .container {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    grid-gap: 10px;
  }

  .item {
    border: 1px solid black;
    padding: 10px;
  }
</style>

<div class="container">
  <div class="item">项目1</div>
  <div class="item">项目2</div>
  <div class="item">项目3</div>
  <div class="item">项目4</div>
  <div class="item">项目5</div>
  <div class="item">项目6</div>
  <div class="item">项目7</div>
  <div class="item">项目8</div>
  <div class="item">项目9</div>
</div>