If you want to quickly set up the environment and try out this gem then you can follow these steps.
docker run -it --name ffi_generator ubuntu bash
# continue in the container
# prepare container
apt update
apt install build-essential
apt install wget
# install llvm
wget https://apt.llvm.org/llvm.sh
chmod +x llvm.sh
./llvm.sh 15 all
# install the packages it suggests then run again
# install ruby
apt install ruby ruby-dev
gem install bundler
# install gem
gem install ffi_generator
# you can try using ffi_generator now
# for running tests
apt install libcairo-dev