Restructured docker building

I have restructured the docker build process entirely, the binary gets
built outside of the docker build command, now we are managing all
dependencies with real Alpine packages and I have dropped features like
socat or the cron daemon.

Signed-off-by: Thomas Boerger <tboerger@suse.de>

docker/etc/s6/openssh/run

@@ -0,0 +1,6 @@
+#!/bin/bash
+[[ -f ./setup ]] && source ./setup
+
+pushd /root > /dev/null
+    exec gosu root /usr/sbin/sshd -E /var/log/sshd.log -D
+popd