default button width in column flex direction bootstrap
<body>
<div class="container">
<div class="row pt-4" id="cards">
<div class="card-deck">
<div class="card">
<div class="card-body d-flex flex-column">
<div> <!-- Just wrap anchor/button in another div -->
<a href="#" class="btn btn-primary mt-auto">Set a Goal</a>
</div>
</div>
</div>
</div>
</div>
</div>
</body>