Update or modernize backend technology

[pascalwengerter]

Currently, this project uses Ruby v2.7.6 with Sinatra inside a "fat" (~500MB IIRC) Docker base image for what is effectively

• a single get endpoint to serve the frontend
• a single post endpoint that performs a handful of syscalls around some tempfiles

Going forward, I see two options:

1. Updating Ruby (to >v3.1.x), Sinatra and using an alpine Ruby base image

2. Switching to hosting the frontend statically and porting the backend to a different technology (Go, Rust, Elixir, Crystal, Gleam, ...). Advantages that this would bring are:

• lower memory footprint in development & production
• (potentially) single binary deployment
• quicker development & deployment loop
• gaining the ability to serve requests concurrently
• (potentially) easier local setup