結論から言うと、公式サイトの通りでOKでした。
# install lib
$ sudo apt-get install -y git curl libssl-dev libreadline-dev zlib1g-dev
# install rbenv
$ git clone https://github.com/rbenv/rbenv.git ~/.rbenv
$ cd ~/.rbenv && src/configure && make -C src
$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc
$ ~/.rbenv/bin/rbenv init
$ echo 'eval "$(rbenv init -)"' >> ~/.bashrc
# restart terminal
# install "rbenv install"
$ mkdir -p "$(rbenv root)"/plugins
$ git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build
# check
$ curl -fsSL https://github.com/rbenv/rbenv-installer/raw/master/bin/rbenv-doctor | bash