Fix builds for Rails integration tests

A couple of things changed recently that broke these for us:

- Rails 7.2 beta dropped support for Ruby < 3.1
- ffi dropped support for Rubygems < 3.3.22

To fix this we have switched from fetching the Rails version to
supplying them in the GitHub actions, at least until Rails 7.2 is out.

We have also updated Rubygems and bundler to new enough versions to
compile the ffi gem.

We also had to add the build-essential package into a later build step
as ffi seems to get built a second time - this is possibly incorrect and
will need looking into.

We should possibly decide to at least drop testing for end of life versions:

- Ruby 2.7.8
- Rails 5.2.8.1

.github/workflows/rails-integration-tests.yml

@@ -18,7 +18,7 @@ jobs:
         name: Compute outputs
         run: |
           rails_versions=$(curl https://rubygems.org/api/v1/versions/rails.json | jq 'group_by(.number[:1])[] | (.[0].number) | select(.[:1]|tonumber > 4)' | jq -s -c)
-          echo "RAILS_VERSIONS=$rails_versions" >> $GITHUB_OUTPUT
+          echo 'RAILS_VERSIONS=["5.2.8.1","6.1.7.8","7.1.3.4"]' >> $GITHUB_OUTPUT
   build-rails:
     strategy:
       fail-fast: false
@@ -114,4 +114,4 @@ jobs:
                contains(needs.*.result, 'failure')
             || contains(needs.*.result, 'cancelled')
             || contains(needs.*.result, 'skipped')
-          }}
+          }}

Dockerfile

@@ -15,6 +15,10 @@ RUN apt-get update -qq && \
 # Install Nokigiri version that supports Ruby 2.7.8
 RUN gem install nokogiri -v 1.15.6
 
+# add a newer version of Rubygems and Bundler
+RUN gem install bundler:2.3.22
+RUN gem update --system 3.3.22
+
 # Install Rails
 ARG RAILS_VERSION=7.1.3.2
 RUN gem install rails -v ${RAILS_VERSION}
@@ -35,7 +39,7 @@ FROM base
 
 # Install packages needed for running the tests
 RUN apt-get update -qq && \
-    apt-get install --no-install-recommends -y curl git && \
+    apt-get install --no-install-recommends -y build-essential curl git && \
     rm -rf /var/lib/apt/lists /var/cache/apt/archives
 
 # Copy built artifacts: gems, application