JSFiddle - React, Tailwind, and code Playground

HTML

<div id="rightDock">
  <div class="flexcontainer">
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
    <div class="flexitem"></div>
  </div>
</div>

<button onclick="addMore();"style="margin: 30px 30px; height: 60px; width:200px">
  Add more divs to flexbox
</button>

CSS

.flexcontainer {
  display: -webkit-flex;
  display: flex;
  -webkit-flex-flow: column wrap;
  flex-flow: column wrap;
  -webkit-align-content: flex-end;
  align-content: flex-end;
  height: 100%;
}

#rightDock {
  position: fixed;
  right: 0px;
  top: 0px;
  background-color: yellow;
  width: auto;
  min-width: 10px;
  height: 100%;
}

.flexitem {
  width: 180px;
  height: 40px;
  margin-right: 15px;
  margin-left: 15px;
  margin-top: 20px;
  background-color: red;
}

JavaScript

function addMore() {
  var element = document.createElement('div');
  element.classList.add("flexitem");
  document.getElementsByClassName('flexcontainer')[0].appendChild(element);
}