Constant ENV_HOST_USER 

pub const ENV_HOST_USER: &str = "FLODL_INTERNAL_HOST_USER";

Controller’s OS user name (resolved on the host by fdl-cli, before any docker spawn). The launcher in the container reads it as the default ssh -l target when the per-host ssh.user: is unset. Bridges the container-vs-host user mismatch (containers ship a stock ubuntu UID-1000 user, but ubuntu@<remote> is rarely the account the user actually uses on cluster hosts).