Add SetCredential= and LoadCredential= to the [Exec] section of the
.nspawn settings file. They take the same arguments as the command line
switches. LoadCredential= is privileged, but SetCredential= is not.
The man page claimed that nspawn propagates the credentials of its
service to the container, which only happens when they are passed on
explicitly, so say so.
Fixes: #38966
Skye Soss
bbca861320