This includes the following version upgrades:

- Docker 18.06.1~ce~3-0~ubuntu -> 5:18.09.2~3-0~ubuntu-bionic
  Fixes CVE-2019-5736
  (http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-5736)
- rke 0.1.15 -> 0.1.16

Closes: #37