<div class="container">
  <button>Button</button>
</div>

.container {
  display: flex;
  
  width: 300px;
  min-height: 2000px;
  margin: 0 auto;

  border: 1px solid #000;
  background: lightskyblue;
}

button {
  position: sticky;
  bottom: 0;
  
  flex-basis: 100%;
  padding: 15px;
  margin-top: auto;

  background: lightsalmon;
  border: none;
}