Can't believe I'm about to say this, but:

Systemd-container is a rather cool SysD extension.

It allows me (Root on most servers) to switch to a customer account in a completely new session, setting all the .profile and .bashrc stuff up, so I can do stuff like control their rootless docker, and no longer have to add my SSH key to their authorized_keys file then re-login under their user.

Nice.