Render a single Jekyll collection only - no other files at all - jekyll

Question

I bought a simple Jekyll template for a support site and have that up and running no issues.
I am trying to render my existing docs collection with separate layout. For background I am taking these docs and making a stripped down html file that has just the body content to use in a small modal pop-up window.
In my config file I have this:
defaults:
# _docs
-
scope:
path: ""
type: docs
values:
layout: doc
which renders my regular HTML files fine along with all my other template content in the target directory.
I am looking to also process this:
defaults:
# _docs
-
scope:
path: ""
type: docs
values:
layout: modal
The issue is that it seems in Jekyll can't do this out of the box. If add this to my config file it only uses the last one. If I try multiple config files they over write each other. I just want my full HTML docs using the doc layout in /docs and the HTML partial files rendered with the same docs collection using the modal layout in /modals as detailed in my previous post Multiple layouts for a single Jekyll collection?.
What I would like to do now is make a separate config file that builds ONLY the docs (using the modals layout) and NOT any of the other files (index, search, posts, 404 etc.) - just the docs collection with the alternate layout. I want the target folder to be the /modals sub directory of my main Jekyll.

Answer 1

This my hack so far - I won't even accept my own answer as it's not really a solution:
I have a second identical config file with this:
keep_files: ["docs"]
collections:
docs:
output: true
permalink: /modals/:name
I run this after the running the main config file. Keeps the normal docs build then adds the custom modal version.