Q:

Não consigo deixar as imgs em ROW com flexbox

Nao consigo colocar elas em row, apenas se eles estiverem width pra ficar todas do lado da outra. 

  

 *{
box-sizing: border-box;
}
 
#countainerslider{
display: flex;
flex-flow: row wrap;
width: 100%;
height: 560px;
}
img{
width: 600px;
height: 200px;;
}
<body>
<div id='page'>
    <header>
    
        <div id="containerimgs">
            <div id="containerslider">
                <img class="imgnav" src="imgs/image3.jpg" alt="image-meat-tomates">
                <img class="imgnav" src="imgs/pepperonipizza.jpg" alt="pepperonipizza">
                <img class="imgnav" src="imgs/strawberry-chicken-salad-with-pecans.jpg"
                    alt="strawberry-chicken-salad-with-pecans">
                <img class="imgnav" src="imgs/burger.jpg" alt="burguer-bacon">
                <img class="imgnav" src="imgs/hotdogg.jpg" alt="hotdog">

            </div>
        </div>
    </header>
    <main>
        <section id="service">

        </section>
        <section id="food">

        </section>
    </main>
    <footer>
        <h3>Follow us :</h3>
        <i></i>
        <i></i>
    </footer>
</div>
</body>

A:

Editei a resposta, simplificando o exemplo.

.row-nowrap {
flex-flow: row nowrap;
}

/* Flex Container */
.container {
margin: 0 auto;
display: flex;
border: 1px solid #ccc;
}
<section class="container row-nowrap">
<img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates">
<img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates"><img class="imgnav" src="http://www.reasoft.com/tutorials/web/img/progress.jpg" alt="image-meat-tomates">
</section>

