Centering content horizontally using flexbox

Remove flex-direction:column and max-width:500px. Give some fixed width say 100px and it will work properly. If you are using flex-direction:column use align-items:center property to horizontally center and justify-center if it is row.

Add align-items:center;

.container {
  display: flex;
  flex-direction: column;
  width: 600px;
  background-color: blue;
  justify-content: center; 
  align-items:center;
}

The property justify-content:center aligns the flex-items along the flex direction - use align-items:center- see demo below:

.container {
  display: flex;
  flex-direction: column;
  width: 600px;
  background-color: blue;
  justify-content: center;
  align-items:center;/*NEW*/
}
.item {
  max-width: 500px;
  width: 250px;/*NEW*/
  height: 100px;
  background-color: yellow;
  border: 2px solid red;
}

<div class="container">
  <div class="item"></div>
  <section class="item"></section>
  <section class="item"></section>
  <footer class="item"></footer>
</div>

Refer: Flexbox W3C spec

Below is an image that applies when flex-direction is row:

a. Main axis alignment : justify-content

b. Cross axis alignment : align-items

When the flex-direction is row, the main-axis is horizontal but when it is column, it is the other way around.