body, html { margin: 0; min-height: 100vh; display: flex; flex-direction: column;}

.container {
  text-align: center;
  width: 100%;
  background: #ccc;
  flex: 1;
}

.btn button {
  background: blue; width: 100%;
  position: fixed;
  bottom: 0;
  left: 0;
  padding: 30px;
  box-sizing: border-box;
}