capistrano-sidekiq systemd rbenv solution. Default configs did not work for me, also there is no proper documentation, Here is what I did:

capistrano-sidekiq systemd rbenv manual.txt

Copy sidekiq.service.capistrano.erb template from "capistrano-sidekiq" gem to "app/config/deploy/templates"

Rbenv default bundler path differs from default '/usr/local/bin/bundler'. 
So you have to set "bundler_path" option in "app/config/deploy.rb".
Here is mine:

set :bundler_path, "/usr/local/rbenv/bin/rbenv exec bundle"

And don't forget to add option to switch to systemd

set :init_system, :systemd

There is two ENV options that needs to be set in sidekiq.service.capistrano.erb

Environment=RBENV_VERSION=<%= fetch(:rbenv_ruby) %>
Environment=RBENV_ROOT=/usr/local/rbenv

You can set RBENV_ROOT with fetch. It's up to you.
Final version:

#############################sidekiq.service.capistrano.erb##############################
[Unit]
Description=sidekiq for <%= "#{fetch(:application)} (#{fetch(:stage)})" %>
After=syslog.target network.target

[Service]
Type=simple
Environment=RAILS_ENV=<%= fetch(:rails_env) %>
Environment=RBENV_VERSION=<%= fetch(:rbenv_ruby) %>
Environment=RBENV_ROOT=/usr/local/rbenv
WorkingDirectory=<%= fetch(:deploy_to) %>/current
ExecStart=<%= fetch(:bundler_path, '/usr/local/bin/bundler') %> exec sidekiq -e <%= fetch(:rails_env) %>
ExecReload=/bin/kill -TSTP $MAINPID
ExecStop=/bin/kill -TERM $MAINPID

RestartSec=1
Restart=on-failure

SyslogIdentifier=sidekiq

[Install]
WantedBy=default.target

#############################/sidekiq.service.capistrano.erb##############################

To make capistrano use my config file, I've created rake file in lib/capistrano/tasks/sidekiq.rake
with following content:

#############################sidekiq.rake##############################
namespace :sidekiq do
  task :install do
    on roles fetch(:sidekiq_roles) do |role|
      switch_user(role) do
        case fetch(:init_system)
        when :systemd
          create_systemd_template
          execute :systemctl, "--user", "enable", fetch(:service_unit_name)
        end
      end
    end
  end

  def fetch_systemd_unit_path
    home_dir = capture :pwd
    File.join(home_dir, ".config", "systemd", "user")
  end

  def create_systemd_template
    search_paths = [
      File.expand_path(
        File.join(*%w[.. .. .. .. config deploy templates sidekiq.service.capistrano.erb]),
        __FILE__
      ),
    ]
    template_path = search_paths.detect {|path| File.file?(path)}
    template = File.read(template_path)
    systemd_path = fetch(:service_unit_path, fetch_systemd_unit_path)
    execute :mkdir, "-p", systemd_path
    upload!(
      StringIO.new(ERB.new(template).result(binding)),
      "#{systemd_path}/#{fetch :service_unit_name}"
    )
    execute :systemctl, "--user", "daemon-reload"
  end
end
#############################/sidekiq.rake##############################

There is only one difference to original deploy config: Path to custom sidekiq.service.capistrano.erb file.

Now sidekiq working like a charm) 
Don't know why there is no working solution for capistrano-sidekiq-systemd-rbenv out of the box.
Feel free to use or create pull request with config generator to original capistrano-sidekiq repo.
Good luck!

Thanks for the tips, but I success to deploy buy simply setting bundler_path in config/deploy.rb (I do not need to override default template and to add env variables RBENV_VERSION and RBENV_ROOT)

set :bundler_path, '/home/orlovic/.rbenv/shims/bundle' 

Thanks for the tips, but I success to deploy buy simply setting bundler_path in config/deploy.rb (I do not need to override default template and to add env variables RBENV_VERSION and RBENV_ROOT)

set :bundler_path, '/home/orlovic/.rbenv/shims/bundle' 

I'm sure if this works to not! Didn't work for me!