CI: make docker volume always succeed in startup

Some CI users (OAI developers) expressed that the warning of the `docker
volume prune` before starting RFsim is confusing. To avoid the warning in
the HTML, force the removal; if the volume does not exist, no warning is
emitted.

To still catch possible problems, do not do this in the `terminate.xml`
so we might still see if there are other problems.