We’ll need to see more than a few other folks who can’t use the node package echo that feature request, before we’ll be able to consider adding it to the build image, since we try very hard to keep it small and have only commonly needed tools “baked in” vs “installed”.
For a quicker-results workaround, you could compile or find a binary that works in ubuntu 16.04 x86_64 and add it to your repo and run directly (./asciidoctor && …)
I had the same problem. However, since asciidoctor is simply a ruby gem and the Netlify build process installs gems before building, this is actually really easy to solve:
Create a Gemfile at the root of your repo (a textfile with the name Gemfile)