I have an HTML file (index.html) in the public folder. These HTML has some "hooks" in it. Like:
<div>{client_ssnumber}</div>
<div>{client_company}</div>
I have to retrieve this file and complete the information in the hooks using data obtained in a controller´s method, then display in the screen.
What is the rails way to do it?
You can't do this (without terrible terrible hacking), it's hard coded to look for public files before checking the router.
Anyway, Your client is wrong. Tell them to use "<%= ... %>" instead of "{ ... }" and move it into a view (you should move it into the view, so it's where you want it to be, then just tell them what the name of the file is).
There is no sensible reason to make up your own templating language. It will be buggier and slower, and it will add a lot of time to get the product out the door. Plus you'll then have to maintain that code. This is a solved problem, use ERB. If they really like that syntax, use Mustache which is pretty similar, and is an existing templating language.
If you can't get them to do this, you have much bigger problems than how to render this page.