Use bundler instead of directly using Rack in the example usage.
Use bundler instead of directly using Rack in the example usage.

Rack is no longer installed in this image by default.

remi/ruby-docker,dazuma/ruby-docker,GoogleCloudPlatform/ruby-docker,dazuma/ruby-docker,GoogleCloudPlatform/ruby-docker,GoogleCloudPlatform/ruby-docker,GoogleCloudPlatform/ruby-docker,dazuma/ruby-docker,dazuma/ruby-docker