The intent is to set up for an upgrade of Ruby and Rails to the modern versions, but I want to start by having a stable running copy that we can incrementally pull up to new versions of things!
And it turns out getting Ruby 1.9.3 to build on modern platforms is hard! I started by trying on macOS and just couldn't get there, the instructions I found for workarounds didn't seem to work anymore.
So the solution I landed on was to set up an Ubuntu VM, and follow some instructions from https://stackoverflow.com/q/51986932 to patch Ruby to work with the version of OpenSSL we have access to!
And it was enough of a challenge that I figured that, rather than setting up the Vagrantfile elsewhere, it would be helpful documentation to do it here, even if we scrap the Vagrantfile etc later once we're in a new stable environment.