pub fn resolve_local_user() -> Option<String>Expand description
Resolve the controller’s OS user name. Used to pre-populate
ENV_HOST_USER before docker spawn, so the launcher inside the
container can default ssh -l <user> to the host’s identity.
Falls through USER then whoami; None when both fail — the
caller then leaves ENV_HOST_USER unset and the launcher omits
-l entirely, so ssh applies its own defaults (ssh_config User
directives, the effective uid). Never fabricate a name: a made-up
-l unknown-user produced a bare “Permission denied (publickey)”
with no hint the username was invented.