make a button appear when hover over a image

Question

I have an image, and I want that when I hover over the image a black background with little opacity cover the image and a button on top of the image appear. Write now I put the button in the place where I want to but I can't make it disappear and appear when I hover.

<div class="container">
   <img src="img_snow.jpg" alt="Snow">
   <button class="btn">Button</button>
</div>

.container {
    position: relative;

    button {
       position: absolute;
       top: 60%;
       left: 50%;
       transform: translate(-50%, -50%);
       -ms-transform: translate(-50%, -50%);
    } 

 button:hover {
    background-color: black;
  }
}

img {
   background-image: url(${({ src }) => src});
   background-repeat: no-repeat;
   background-size: cover;


   &:hover {
      background-color: #000;
      opacity: 0.5;
  }
}

Answer 1

I have used a pseudo element on the .container element with a background color with opacity. Rather than hovering over the image, I have shown the button when hovering over the container.

.container {
    position: relative;  
    width:100%;
    max-width:100px;
}
.container:before {
  content:"";
  position:absolute;
  width:100%;
  height:100%;
  top:0;left:0;right:0;
  background-color:rgba(0,0,0,0);
}
.container:hover::before {
  background-color:rgba(0,0,0,0.5);
}
.container img {
  display:block;
}
.container button {
  position: absolute;
  top: 60%;
  left: 50%;
  transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%);
  opacity:0;
} 
.container:hover button {   
  opacity: 1;
}

<div class="container">
  <img src="http://placeimg.com/100/100/animals" alt="Snow">
   <button class="btn">Button</button>
   
</div>

Answer 2

Use a pseudo element on the container and start it at 0 opacity. Same with the button. Then on hover of the container you change the opacity on those elements to make them appear. Transition property makes for a nice finishing touch on the interaction.

.container:before {
  opacity: 0;
  background: rgba(0, 0, 0, 0.5);
}
.container:hover:before {
  opacity: 1;
}

And the button same thing:

.btn {
  opacity: 0;
}
.container:hover .btn {
  opacity: 1;
}

.container {
  position: relative;
  width: 400px;
  height: 220px;
  border: 2px solid black;
  display: flex;
  align-items: center;
  justify-content: center;
}
.container:before {
  /* empty pseudo */
  content: '';
  
  /* start transparent, include a transition bcuz */
  opacity: 0;
  transition: opacity 0.5s ease;
  
  /* covers the whole div */
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: rgba(0, 0, 0, 0.5);
  z-index: 2;
}
.container:hover:before {
  opacity: 1;
}
.container img {
  position: absolute;
  display: block;
  max-width: 100%;
  height: auto;
  z-index: 1;
}

.btn {
  opacity: 0;
  transition: opacity 0.5s ease;
  position: relative;
  padding: 0 40px;
  height: 40px;
  line-height: 40px;
  max-width: 260px;
  cursor: pointer;
  z-index: 3;
}
.container:hover .btn {
  opacity: 1;
}

<div class="container">
  <img src="https://picsum.photos/400/220" alt="Snow">
  <button type="button" class="btn">Click Me</button>
</div>