Add syslog & fix /data/git rights & SOCAT_LINK env var

- Resolve #1893
- Add syslogd to output sshd log on stdout (via `docker logs`)
- Enforce directory rights on `/data/git`, `/data/gogs` & `/data/ssh`
- Add `SOCAT_LINK` environment variable to prevent the creation of scout links when they are not needed (see #1815)

docker/s6/openssh/setup

@@ -23,4 +23,5 @@ fi
 
 # Set correct right to ssh keys
 chown -R root:root /data/ssh/*
-chmod 600 /data/ssh/*
+chmod 0700 /data/ssh
+chmod 0600 /data/ssh/*