<< CSS >>
.header {
    height: 100vh;
    background-color: #1e3749;
    z-index: 0;

}
.bg {
    height: 100%;
    background-image: url("/Moja/img/bg2.png");
    background-repeat:no-repeat;
    background-size:contain;
    background-position:left bottom;
    background-color: transparent;
    z-index: 10;
}
.header-text {
    position: fixed;
    padding-top: 30vh;
    z-index: 0;
}

<< HTML >>
<div class="container-fluid">
    <div class="header">
            <div class="text-center header-text">
                <h1>Test test</h1>
                <p>To tylko zabawa</p>
            </div>
            <div class="text-center bottom-align-text">
                <i class="fa fa-arrow-down fa-3x arrow" aria-hidden="true"></i>
            </div>
        <div class="bg"></div>
    </div>
</div>