Okay, so this isn't the minimum viable Ruby server, but it is actually useful. Put static files in /public and our server will serve them. Put .html.erb files in /views directory and our server will add a layout to them and serve them at the same path. So a file in /views/me.html.erb will be located at localhost:8000/me.
This server uses pure Ruby and standard lib, no install required. Just have vanilla Ruby on your machine.
Start the Server
$ ruby server.rb
Visit localhost:8000
We added a blank Gemfile so Heroku knows to install Ruby on a dyno, then we added a Procfile so Heroku knows how to start our server.
$ heroku create
$ git push heroku master
See it in action at minimumviablerubywebsite.heroku.com
Why would you ever want to do this? There are plenty of web frameworks like Rails and Sinatra out there, and more performant web servers than webrick, but it is easy to forget how powerful and flexible pure Ruby is especially when you add in the standard library. You could use this project as a stepping stone to building your own web framework, or learning how those other frameworks work their magic.