How to display hr horizontally with flex?

Use flex-grow: 1 or width: 100% so it will grow to match the width of the parent. You probably want to use that in combination with flex-wrap: wrap on .container if you plan on putting more flex children in .container

.container {
  display: flex;
  flex: 1;
}

hr {
  flex-grow: 1;
  /* width: 100%; */ /* or this */
}

<div class='container'>
    <hr>
</div>