Button relative to scroll-content background

I have a background image and have set this image at scroll-content class.
I want to make a button that stand always in the end of background.
Until know with some css is possible to postion but when change from pc to mobile it change.
I want something that work for all platforms.

<ion-col col-2></ion-col>
<ion-col col-8 style="text-align: center;">     
    <button class="btn" ion-button block large (click) = login()>ENTRA</button>   
<ion-col col-2></ion-col>

.scroll-content {
background: url("…/imgs/Sfond.svg") no-repeat center center / cover;
margin-top: -6px;
bottom: 55px;
margin-right: -100px;
margin-left: -100px;
margin-bottom: -3px !important;
position: absolute;

Thank you in advance.