Cant add border to Button using XML : <p>I just wanted to add a broder to my Button but its not working.</p>

<p>Main xml script:</p>

<p><a href="https://i.stack.imgur.com/yg7cT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yg7cT.png" alt="enter image description here"></a></p>

<p>button_style4.xml:</p>

<p><a href="https://i.stack.imgur.com/YlSN7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YlSN7.png" alt="enter image description here"></a></p>

<p>The Button appears normaly in the App and it works as I want it to work just the border is not appearing...
Someone find the fault?</p>
