SSH Remote filesystem spurious failures on first run even with proper known_hosts

[ml-evs]

It's hard to test, but I think even with a hostkey stored in known_hosts, the first time a container tries to SSH, the process spits out a message to stderr which we treat as a fatal error. Instead, we should check for the string "Permanently added <xx.yy> to the list of known hosts" and allow that through.