Ensure compatible FFI version

We load and test multiple Ruby and Rails versions and cannot always rely
on having the latest versions of dependencies.

We started seeing errors with FFI 1.17.x because it requires Ruby Gems
3.3.22.

Like Nokogiri, we'll install a known compatible version for our oldest
set of dependencies.

Dockerfile

@@ -15,6 +15,9 @@ RUN apt-get update -qq && \
 # Install Nokigiri version that supports Ruby 2.7.8
 RUN gem install nokogiri -v 1.15.6
 
+# Install FFI that supports Ruby gems < 3.3.22
+RUN gem install ffi -v 1.16.3
+
 # Install Rails
 ARG RAILS_VERSION=7.1.3.2
 RUN gem install rails -v ${RAILS_VERSION}