Install RVM

curl -L get.rvm.io > rvm-install
bash < ./rvm-install

For bash completion add to .bashrc:

# RVM bash completion
[[ -r "$HOME/.rvm/scripts/completion" ]] && source "$HOME/.rvm/scripts/completion"

If necessary in $HOME/.rvm/scripts/completion replace line 22 with:

source "$HOME/.rvm/scripts/extras/completion.bash"

Set the terminal as login terminal in preferences.

In VSCode add to settings.json:

"terminal.integrated.shellArgs.linux": [ "-l" ]

Install Ruby

rvm install 2.5.3
rvm use 2.5.3 --default