Der Container ist ganz oben was auch immer man probiert
<div class="container mx-auto">
<div class="row">
<div class="col-sm-3"></div>
<div class="col-sm-6">
<div class="card">
<div class="card-header">Header</div>
<div class="card-body">Content</div>
<div class="card-footer">Footer</div>
</div>
</div>
<div class="col-sm-3"></div>
</div>