fix: make sure bundler version for gem-update.sh matches bundler version of images

gem-update.sh

@@ -6,7 +6,7 @@ docker run --rm -it -v "$(pwd)":/app \
   -c "apt-get update; \
       apt-get install -y cmake build-essential ruby-dev libpq-dev default-libmysqlclient-dev; \
       cd /app; \
-      gem install --default bundler:2.4.12; \
+      gem install --default bundler:2.4.18; \
       gem update --system; \
       bundle update --bundler; \
       bundle outdated; \