How about using 0 width pseudo elements before and after the items?
.container{
display: flex;
flex-wrap: wrap;
justify-content: space-between;
border: 1px solid #C55;
}
.column {
flex: 1 20%;
max-width: 20%;
height: 100px;
background: #C55;
}
.even:before, .even:after{
content: '';
display: block;
width: 0;
}
no side spacing:
side spacing: