[atomic] "LABEL install/uninstall" guidelines

[Ken Dreyer <kdreyer redhat com>]

Are there any guidelines or policies for what to put in the "install"
or "uninstall" labels in my Dockerfiles? Can images basically do
whatever they want in those scripts?

I see examples where the install script sets up systemd unit files -
is that the pattern these install shell scripts should always follow?

I also saw an example where an image provides "LABEL install" but no
"LABEL uninstall". Is that bad?

- Ken