Dockerfile: Add torsocks binary.

This has two purposes:
1) The 'torify' wrapper is already included, and it needs torsocks.
2) Use torsocks for health checking your onion service like this:

healthcheck:
  test: ["CMD-SHELL", "torsocks nc -z <onion address> <port> || exit 1"]
  interval: 2m

last_torsocks_version.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+git ls-remote --tags https://git.torproject.org/torsocks.git | grep -oE 'v[0-9]+\.[0-9]+\.[0-9]+$' | sort -V | tail -1