Preventing Bootstrap button from filling flexbox
To you button class, add mx-auto to center the button and mt-auto to push the button at the bottom of the card.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<div class="row pt-4" id="cards">
<div class="card-deck">
<div class="card">
<img class="card-img-top img-fluid" src="{{url_for('static', filename='volunteer.png')}}" alt="exercise_image">
<div class="card-body d-flex flex-column align-items-center">
<h5 class="card-title">Giving Back</h5>
<p class="card-text">Help others and feel useful</p>
<a href="#" class="btn btn-primary mt-auto mx-auto">Set a Goal</a>
</div>
</div>
</div>
</div>
</div>
</body>
</html>
Just wrap your anchor in another div
<div>
<a href="#" class="btn btn-primary mt-auto">Set a Goal</a>
</div>
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<div class="row pt-4" id="cards">
<div class="card-deck">
<div class="card">
<img class="card-img-top img-fluid" src="{{url_for('static', filename='volunteer.png')}}" alt="exercise_image">
<div class="card-body d-flex flex-column">
<h5 class="card-title">Giving Back</h5>
<p class="card-text">Help others and feel useful</p>
<div>
<a href="#" class="btn btn-primary mt-auto">Set a Goal</a>
</div>
</div>
</div>
</div>
</div>
</div>
</body>
</html>OR
Just add align-items-center class on the flex div
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<div class="row pt-4" id="cards">
<div class="card-deck">
<div class="card">
<img class="card-img-top img-fluid" src="{{url_for('static', filename='volunteer.png')}}" alt="exercise_image">
<div class="card-body d-flex flex-column align-items-center">
<h5 class="card-title">Giving Back</h5>
<p class="card-text">Help others and feel useful</p>
<a href="#" class="btn btn-primary mt-auto">Set a Goal</a>
</div>
</div>
</div>
</div>
</div>
</body>
</html>