如何让物品伸展以填充宽度？

Question

.container {
  display: grid;
  grid-template-columns: repeat(auto-fill, 100px);
  grid-gap: 3px;
  border: 1px solid green;
}

.item {
  border: 1px solid red;
  height: 50px;
}

<div class="container">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

我怎样才能将红色item盒子拉伸一点，以便它们填满绿色container盒子的整个宽度？

它们应该都是相同的宽度，并且大于100px。

最后一行应该像现在一样左对齐。