Como faço o download de arquivos com Capistrano v3

TL; DR

on roles(:backup) do |host|
    download! "backup/backup.tgz", "backup-#{fetch(:host})}.tgz"
end

ATUALIZAÇÃO: Encontrei e comecei a usar o método SSHKit download!assim run_locallycomo sugerido. Como o Capistrano V3 está no SSHKit, isso deve ser uma queda em substituição à maneira que eu estava usando system.

namespace :backup do
    desc "Package a backup file, download it to the local system, and extract locally"
    task "files" do
        on roles(:db) do |host|

            # create the backup somehow on the remote server
            within "/backup" do
                execute :tar, "-czvf backup.tgz path_to_file_to_backup.txt"
            end

            # download the backup file
            on roles(:backup) do |host|
                download! "/backup/backup.tgz", "backup-#{fetch(:host})}.tgz"
            end

            # and unpack the tgz backup file
            run_locally do
                execute :tar, "-xzvf backup-#{fetch(:host})}.tgz"
            end

        end
    end
end

Este é o método antigo com notas. Usar systeme chamar manualmente scpnão parecia a melhor maneira, mas funcionou. Mantendo-o aqui para referência.

namespace :backup do
    desc "Package a backup file, and download it to the local system.""
    task "files" do
        on roles(:db) do |host|
            # create the backup somehow on the remote server
            within "~/backup" do
                execute :tar, "-czvf backup.tgz path_to_file_to_backup.txt"
            end

            # download and unpack the tgz backup file
            # note (bins): currently this requires scp and tar to be
            # available on your local system
            #
            # note (auth): this scp command is assumed to work because
            # you can access the deploy user already via your public/private
            # key, since otherwise 'cap' commands via sshkit would not
            # work it is of course possible that you'll need to tweak
            # this if thats not the case for you
            # system runs commands locally, so here we are just
            # automating the process of fetching a remote file via scp
            #
            # use the variables we have accessible via the cap env to scp
            # remote file to local system
            # file name would be in this case something like
            # "backup-example.com.tgz" if the :host was example.com
            system("scp #{fetch(:deploy_user)}@#{host}:~/backup.tgz backup-#{fetch(:host})}.tgz")

            # furthermore, you could run commands locally to unpack
            # or move the file into location
            system("tar -xzvf -#{fetch(:host})}")
            system("rm -f -#{fetch(:host})}")
            system("mv -f path_to_file_to_backup.txt data/backup/backup.txt")
        end

    end
end