$container = get_theme_mod( 'understrap_container_type' );

add_filter( 'theme_mod_understrap_container_type', 'override_container_type' );
function override_container_type() {
    if ( get_field('sidebar_position') !== 'inherit' ){
        return get_field('sidebar_position');
    }
    else {
        return get_theme_mod( 'understrap_sidebar_position' );
    }
}