capistrano/docs/documentation/getting-started/local-tasks/index.markdown

title	layout
Local Tasks	default

Local tasks can be run by replacing on with run_locally:

desc 'Notify service of deployment'
task :notify do
  run_locally do
    with rails_env: :development do
      rake 'service:notify'
    end
  end
end

Of course, you can always just use standard ruby syntax to run things locally:

desc 'Notify service of deployment'
task :notify do
  %x(RAILS_ENV=development bundle exec rake "service:notify")
end

Alternatively you could use the rake syntax:

desc "Notify service of deployment"
task :notify do
   sh 'RAILS_ENV=development bundle exec rake "service:notify"'
end