Add ENV RUBYGEMS_VERSION and set to latest 3.5.x

that is GitLab v17.0 recommendation

MR that updates docs/install/installation.md:
- "Bump version of rubygems in installation instructions"
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149976

There is no entry in .tools-version

Dockerfile

@@ -5,6 +5,7 @@ ARG VERSION=17.0.2
 ENV GITLAB_VERSION=${VERSION} \
     RUBY_VERSION=3.2.4 \
     RUBY_SOURCE_SHA256SUM="c72b3c5c30482dca18b0f868c9075f3f47d8168eaf626d4e682ce5b59c858692" \
+    RUBYGEMS_VERSION=3.5.14 \
     GOLANG_VERSION=1.22.4 \
     GITLAB_SHELL_VERSION=14.35.0 \
     GITLAB_PAGES_VERSION=17.0.2 \

assets/build/install.sh

@@ -56,6 +56,9 @@ make -j"$(nproc)"
 make install
 cd "$PWD_ORG" && rm -rf /tmp/ruby
 
+# upgrade rubygems on demand
+gem update --no-document --system "${RUBYGEMS_VERSION}"
+
 # PaX-mark ruby
 # Applying the mark late here does make the build usable on PaX kernels, but
 # still the build itself must be executed on a non-PaX kernel. It's done here