After referencing a tutorial, I implemented the following code:
<div class="card" style="width:400px">
<img class="card-img-top" src="image.png" alt="Card image">
<div class="card-body">
<h4 class="card-title">John Doe</h4>
<p class="card-text">Some example text.</p>
<a href="#" class="btn btn-primary">See Profile</a>
</div>
</div>
This code creates a white border around the image within the grey area. No additional CSS is being applied: