How do I use a variable in a dmx-bind:data-src?

In playing with CDNs, I’d like to simplify my formulas in my source.
So I set variables:
<dmx-value id="cdnprefix2" dmx-bind:value="https://test.imgix.net/"></dmx-value>
<dmx-value id="cdnimages" dmx-bind:value="?w=100"></dmx-value>

Then, in this statement, I’d like to reference my variables:
<img dmx-bind:data-src="cdnprefix2.value+image1+cdnimages.value" >

My desired result is: (the prefix + filename + imagingDirective)
src=“https://test.imgix.net/engine1.jpg?w=100”

but i get:
http://localhost:3000/engine1.jpg
Its like it doesn’t see the variables at all? I’m missing something fundamental.