Apply image overlay when hovering exactly within the image

I have two flexbox columns. The left column contains text; the right column contains a picture. The columns should take up 60% and 40% of the width of the page, respectively. The text is longer than the picture is tall. I'd like a semi-transparent overlay on the picture; when I hover on the overlay, I'd like the overlay to disappear.

However, I can't get the overlay to show up, and I can't seem to restrict the hover area to just the size of the picture -- anywhere in the right column, even below the picture, triggers the hover effect.

Because I want the page to be responsive, I don't want to manually set the picture's size (or aspect-ratio): it should fill the width of the right column, and the right column should fill 40% of the page's width -- no px here, please.

Here's a quick mockup of what I'm trying to do. In this example, the yellow #img-container expands below the bottom of the image, and the green .overlay doesn't show up. Both should be exactly the size of the image.

Fiddle here.

Am I going about this the right way? Is there something obvious I'm missing? I'm pretty new to web design... thanks for the help!

Solution

To be able to put an overlay over a specific element:

you will need the element you are overlaying (your #container-img) to be position: relative.
The overlay itself must be positioned 'relative' to that element with position: absolute.
Depending on your requirements the overlay is positioned inside the element by assigning values to top, right, bottom and/or left or, as I used in the snippet, define their shorthand inset property. As you want to overlay the image, inset: 0 will stretch the overlay to all sides of its parent #container-img.
All that is left to do is define CSS for when #container-img is hovered.

I commented my changes and additions in the snippet:

#container {
  display: flex;
  gap: 50px;
}

#container-text {
  flex: 3;
}

#container-img {
  flex: 2;
  background-color: yellow;
  align-self: flex-start;
}

/*************************/
/* Changes and additions */
/*************************/
/* Changed */
#container-img img {
  display: block; /* removes small whiete space below image */
  /* disable this property to see the difference! */

  width: 100%;
  opacity: 20%;
}

#container-img .overlay {
    content: "";
    position: absolute;
    inset: 0; /* shorthand for T/R/B/L properties */
    background: rgb(0,255,0,.2); /* a semi transparent color */
}

/* ADDED */
#container-img {
  position: relative; /* new stacking context to position overlay */
}
#container-img:hover .overlay {
  display: none; /* hide the overlay on hover */
}

<div id="container">

  <div id="container-text">
    <p>teeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeext</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
    <p>text</p>
  </div>

  <div id="container-img">
    <img src="https://play-lh.googleusercontent.com/IeNJWoKYx1waOhfWF6TiuSiWBLfqLb18lmZYXSgsH1fvb8v1IYiZr5aYWe0Gxu-pVZX3" alt="profile picture">
    <div class="overlay"></div>
  </div>

</div>