jsbin.fixer.css

.container {
  /* removed width here */
  border: 1px solid black;  
  height:80px; 
  overflow-x:auto; 
  overflow-y:hidden; /* turns off y-scrollbar */
  display:block;
}
.image-container {
  background: blue; 
  border:1px solid black; 
  width:50px; 
  height: 50px;
}
/* changed to a div */
.span-container {
  background: green; 
  border:1px solid black; 
  width:50px; 
  height: 50px; 
  float: left;
}

jsbin.fixer.html

<table>
    <thead>
        <tr>
            <th>
                <div class="container">    
                    <div class="span-container">
                        <div class="image-container">
                          <img src="http://jsfiddle.net/img/logo.png"/> 
                        </div>
                        <div>Test</div>
                    </div>
                    
                    <div class="span-container">
                        <div class="image-container">
                          <img src="http://jsfiddle.net/img/logo.png"/> 
                        </div>
                        <div>Test</div>
                    </div>
                 
                  <div class="span-container">
                        <div class="image-container">
                          <img src="http://jsfiddle.net/img/logo.png"/> 
                        </div>
                        <div>Test</div>
                    </div>
                 
                  <div class="span-container">
                        <div class="image-container">
                          <img src="http://jsfiddle.net/img/logo.png"/> 
                        </div>
                        <div>Test</div>
                    </div>
                   
                  <div class="span-container">
                        <div class="image-container">
                          <img src="http://jsfiddle.net/img/logo.png"/> 
                        </div>
                        <div>Test</div>
                    </div>
                                       
                </div>
            </th>
        </tr>
    </thead>
</table>