Setting `Setsid` is needed for SSH connection helper with `ProxyCommand`
config, so as to detach TTY.
e.g.
$ cat ~/.ssh/config
Host foo
Hostname foo
ProxyCommand ssh -W %h:%p bastion
$ DOCKER_HOST=ssh://foo docker run -it --rm alpine
/ #
Fix#1707
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>