How do I create a new rails application with ubuntu and postgresql

I'm currently trying to create a new rails app. I can easily do this with rails new app --database=postgresql but when I change the database.yml, I'm using the same user for every application for example postgres. Therefore all applications share the same database user.

Then when I try to install a gem in my new application it says, that the gem is already installed.

Therefore I created a new user, but this didn't really work. I can create a new rails app with the new user but it throws erros that I need to to install, bundler, that it cant find the bundle folder and so on.

Do have any guidance on this ? I'm using Ubuntu 18.04 on Windows.