`deploy:setup` is successful, yet `deploy:cold` / `deploy:check` fail

deploy.rb

require 'bundler/capistrano'

default_run_options[:pty] = true

set :application, 'example'
# repo
set :scm,         :git
set :repository,  "git@github.com:mariusbutuc/#{application}.git"
set :branch,      'master'
# deploy
set :user,        'vagrant'
set :deploy_to,    "/home/#{user}/#{application}"
set :deploy_via,  :remote_cache
set :ssh_options, { forward_agent: true }

server            'example.com', :web, :app, :db, primary: true

namespace :unicorn do
  desc "Start unicorn for this application"
  task :start do
    run "cd #{current_path} && bundle exec unicorn -c /etc/unicorn/#{application}.conf.rb -D"
  end
end