wagtail_bakerydemo/bakerydemo/templates/search/search_box.html

<form action="{% url 'search' %}" class="form-inline" method="get">
    <div class="row">
        <div class="col-sm-12">
            <div class="input-group">
                <input type="text" name="query" class="form-control" />
                <div class="input-group-btn">
                    <button type="submit" class="btn btn-primary">
                        <i class="glyphicon glyphicon-search"></i> Search
                    </button>
                </div>
            </div>
        </div>
    </div>
</form>