<footer class="footer mt-auto py-3">
    <div class="container ">
        <span class="mt-5 mb-3 text-muted text-center">&copy; STVAT 2017-2020.</span>
    </div>      
</footer>
    <style>
      .container {
  width: auto;
  max-width: 680px;
  padding: 0 15px;
  text-align: center;
}

.footer {
  background-color: #f5f5f5;
  position:fixed;
  bottom:0;
  width:100%;
}
    </style>