Add an sshd container

This is mainly for testing.

You can run with something like
```
docker run -v ~/.ssh/id_rsa.pub:/root/.ssh/authorized_keys -p 2222:22 -e TINI_SUBREAPER=1 --pid=host mobylinux/sshd:36c44542d8120e384c724a078e3e489f7a58382c
```

if you want to run in the host pid namespace. You must provide an
authorized key, you may provide a host key.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

base/sshd/usr/bin/ssh.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+KEYS=$(find /etc/ssh -name 'ssh_host_*_key')
+[ -z "$KEYS" ] && ssh-keygen -A >/dev/null 2>/dev/null
+
+exec /usr/sbin/sshd -D