namespace :a9n do

desc 'Copy stage configuration to the base file.'
task :copy_stage_config do
  on roles(:app) do
    within fetch(:release_path) do
      execute :cp, 'config/configuration.yml.#{fetch(:stage)}', 'config/configuration.yml'
    end
  end
end

end

after ‘deploy:updating’, ‘a9n:copy_stage_config’