How to make borders collapse on a div

Question

This is my code:

<div class="container">
    <div class="column">
        <div class="cell"></div>
        <div class="cell"></div>
        <div class="cell"></div>
    </div>
    <div class="column">
        <div class="cell"></div>
        <div class="cell"></div>
        <div class="cell"></div>
    </div>
    <div class="column">
        <div class="cell"></div>
        <div class="cell"></div>
        <div class="cell"></div>
    </div>
    ...
</div>

Then CSS

.container {
    display: table;
    border-collapse: collapse;
}
.column {
    float: left;
    overflow: hidden;
    width: 120px;
}
.cell {
    display: table-cell;
    border: 1px solid red;
    width: 120px;
    height: 20px;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

Why do the cells that have displayed: table-cell and border-collapse: collapse in the outer div still not collapse? What do I not see here?

By the way, a column may contain a variable amount of cells, therefore I can't have borders on only one side.