Fix Ruby quickstart container startup

[Haihan-Jiang]

Description

Fixes #901.

The Ruby quickstart was still missing the server gems required by current Sinatra. Building the image with the existing Gemfile and starting it in the ruby:3.3-slim container fails because Sinatra 4 cannot find a Rack handler.

This PR adds the missing rackup and puma runtime dependencies, commits a lockfile for reproducible container builds, and runs the app through bundle exec. The Dockerfile installs build tools only for bundle install, then removes them from the final image layer.

Validation:

• docker build -t kes-ruby-quickstart-test quickstarts/languages/ruby
• docker run -d --name kes-ruby-quickstart-test-run -p 18080:8080 -e TARGET=GKE kes-ruby-quickstart-test
• curl -fsS http://localhost:18080/ returned Hello GKE!
• Container logs show Sinatra 4.2.1 running with Puma 8.0.2 in production mode.
• git diff --check

Tasks

• The contributing guide has been read and followed.
• The samples added / modified have been fully tested.
• Workflow files have been added / modified, if applicable.
• Region tags have been properly added, if new samples.
• Editable variables have been used, where applicable.
• All dependencies are set to up-to-date versions, as applicable.
• Merge this pull-request for me once it is approved.