You'll need chruby, and a ruby that is local to your user home directory.
On macOS, it's easy. Jsut brew install chruby and you're done.
On Ubuntu Linux, there's sadly no automated installer. You'll have to run the manual install process, as describe in the Install section of chruby.
In both cases, you'll need to tweak your ~/.bashrc as described in the
Auto-Switching section.
chruby-repo chruby-install chruby-auto-switching
Again it's easy on macOS with brew install ruby-build and your get a recent
version that allows you to build recent ruby versions.
On Ubuntu Linux, that's sightly more complicated because Aptitude repositories
provide versions of ruby-build that are so old that it makes it pointless.
So, you'd better off with installing ruby-build yourself. (This could be
done as a rbenv plugin, but we use chruby instead for its improvements
over rbenv.) So we default to installing ruby-build as a standalone
program, which is the 3rd option of the Installation
process.
cd /opt
git clone https://github.com/rbenv/ruby-build.git
PREFIX=/usr/local ./ruby-build/install.sh
Build your version of Ruby. Check the version mentionned in the .ruby-version
file at the root of your project working tree.
ruby-build 2.4.2 --install-dir ~/.rubies/ruby-2.4.2
Once done, you get your own separate Gem repository for this Ruby version, but you need to update this Gem system.
gem update --system
Then you can download the Gems that are required for your project, as
expressed in the Gemfile. For this, you first need bundler.
gem install bundler
Now you can download your project specific Gems.
bundle install
And you're done. Now you are ready to run your own ruby scripts!