Add ruby image version. Error was "google-protobuf-3.21.12-x86_64-linux...

Add ruby image version. Error was "google-protobuf-3.21.12-x86_64-linux requires ruby version < 3.2.dev, >= 2.5, which is incompatible with the current version, ruby 3.2.0p0"

.gitlab-ci.yml

-image: ruby
+image: ruby:2.6
 
 before_script:
   - gem install bundler:2.1.4