There is not a lot of doc or posts about making an headless renderer with webgl support. Here are few of my findings
I tried things on Heroku, but I was not able to make anything work.
Next, I tried EC2 t2.micro, only to find out that OpenGL needs a graphic card to execute (sure, there are things like mesa that I didn't tried, but it looked like I needed to build Chrome and I didn't want to go that way).
The only easy solution that I found was AWS EC2 GPU Instances. They are pretty much overpriced for what I'm trying to acheive, but it worked.
You need to use an AMI that support the Nvidia GRID thing. eg https://aws.amazon.com/marketplace/pp/B00FYCDDTE and its CentOS.
Slimer need Firefox and Firefox need shared libraries that we need to install (we will use a custom repo).