The problem is that, when running gem install bundler via ansible, you’re not initializing rbenv properly, since rbenv init is run in .bashrc or .bash_profile. So the gem command used is the system one, not the one installed as a rbenv shim. So whenever you install a gem, it is installed system-wide, not in your rbenv environment.
To have rbenv initialized properly, you must execute bash itself and explicitely state that it’s a login shell, so it reads it’s initialization files :
ansible your_host -m command -a 'bash -lc "gem install bundler"' -u your_rbenv_user
Leave the -u your_rbenv_user part if you really want to do this as root.
If the above command works, you can easily turn it into a playbook action :
- name: Install Bundler
become_user: your_rbenv_user
command: bash -lc "gem install bundler"
It’s cumbersome, but it’s the only way I found so far.