调整浏览器大小时对齐Flexbox部分

body,
html {
  margin: 0px;
  font-family: lato-reg;
  font-size: 16px;
  color: #3c3c3c;
  overflow-x: hidden;
}
img {
  width: 100%;
  height: 100%;
}
.section {
  position: relative;
}
/*--------------------------------------Fifth Section--*/

.flex-container {
  margin-top: 10px;
  width: 100%;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-flex-flow: row wrap;
  -ms-flex-flow: row wrap;
  flex-flow: row wrap;
}
.work-indiv {
  width: 540px;
  -webkit-flex: auto;
  -ms-flex: auto;
  flex: auto;
  display: inline-block;
  height: 237px;
  position: relative;
  margin-bottom: 10px;
  cursor: pointer;
  transition: 1s;
}
.work-indiv:hover img {
  opacity: .6;
  transition: .6s;
}
.middle {
  padding-left: 10px;
  padding-right: 10px;
  width: 783px;
}
.work-indiv span {
  position: absolute;
  clear: both;
  bottom: 0;
  left: 0;
  color: White;
  margin-left: 40px;
  text-transform: uppercase;
  margin-bottom: 40px;
  font-size: 2.125em;
  font-family: latobold;
  letter-spacing: .16em;
}
.middle span {
  margin-left: 50px;
}

<div class="section" id="fifth">
  <div class="flex-container">
    <div class="work-indiv">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="Topworkz">
      <span>Topworkz</span>
    </div>
    <div class="work-indiv middle">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="Joint">
      <span>Joint</span>
    </div>
    <div class="work-indiv">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="Market">
      <span>Market</span>
    </div>

    <div class="work-indiv">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="Worktop">
      <span>Worktop</span>
    </div>
    <div class="work-indiv middle">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="Lable">
      <span>Lable</span>
    </div>
    <div class="work-indiv">
      <img src="https://pixabay.com/static/uploads/photo/2015/12/21/06/41/landscape-1102117_960_720.jpg" alt="KYIV">
      <span>KYIV</span>
    </div>


  </div>