What is background_image set to return as? If it’s set to return an array, what you need in for the way you’re using it in the example is the URL. Depending on what size you’re trying to pull, what that looks like changes but it’d likely be something comparable to:
<?php $background = get_field('background_image'); ?>
<div class="fullwidth-container" style="background-image: url(<?php echo $background['url']; ?>);">
Instead of $background[‘url’], it may be something like $background[‘sizes’][‘large’] or comparable.
