chore(ci): Fix logstash build (#18199)

The logstash plugin build step in Github workflows failed https://github.com/grafana/loki/actions/runs/15814913050/job/44576897829



Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

clients/cmd/logstash/Dockerfile

@@ -1,12 +1,12 @@
 FROM logstash:9.0.2
 
 USER logstash
-ENV PATH /usr/share/logstash/vendor/jruby/bin:/usr/share/logstash/vendor/bundle/jruby/2.5.0/bin:/usr/share/logstash/jdk/bin:$PATH
+ENV PATH /usr/share/logstash/vendor/jruby/bin:/usr/share/logstash/vendor/bundle/jruby/3.1.0/bin:/usr/share/logstash/jdk/bin:$PATH
 ENV LOGSTASH_PATH /usr/share/logstash
-ENV GEM_PATH /usr/share/logstash/vendor/bundle/jruby/2.5.0
+ENV GEM_PATH /usr/share/logstash/vendor/bundle/jruby/3.1.0
-ENV GEM_HOME /usr/share/logstash/vendor/bundle/jruby/2.5.0
+ENV GEM_HOME /usr/share/logstash/vendor/bundle/jruby/3.1.0
 
-RUN gem install bundler:2.3.6
+RUN gem install bundler -v 2.6.9
 
 COPY --chown=logstash:logstash ./clients/cmd/logstash/ /home/logstash/
 WORKDIR /home/logstash/

clients/cmd/logstash/Gemfile

@@ -12,3 +12,6 @@ else
 end
 
 gem "webmock", "~> 3.8"
+# I could not find the reason why the `cgi` gem is required after the upgrade of jruby inside the logstash bash Docker image.
+# However, the build fails without this dependency.
+gem "cgi"