(Fix?) Upgrade Rubygems version

Attempting to fix a bundler error on CI in Docker by updating Rubygems version.

Error is:

```
0.405 bundler: failed to load command: rake (/usr/local/bundle/bin/rake)
```

Dockerfile

@@ -37,7 +37,7 @@ ENV BUNDLE_GEM_GROUPS ${RAILS_ENV}
 COPY Gemfile ${DEPS_HOME}/Gemfile
 COPY Gemfile.lock ${DEPS_HOME}/Gemfile.lock
 
-RUN gem update --system 3.3.5
+RUN gem update --system 3.5.1
 RUN gem install bundler -v 2.3.5
 RUN bundle config set frozen "true"
 RUN bundle config set no-cache "true"