This includes the following version upgrades:

- Kubernetes v1.12.3 -> v1.12.5
- rke 0.1.14 -> 0.1.15

[Rancher 1.6.26 supports Docker 18.09](https://github.com/rancher/rancher/releases/tag/v1.6.26),
but rke 0.1.14 stil doesn't, see
https://code.greenhost.net/openappstack/bootstrap/issues/33#note_44119.

Closes: #33

Also:

- Add a .talismanrc file to ignore files beeing scanned by
  Talisman, a "tool that installs a hook to your repository
  to ensure that potential secrets or sensitive information
  do not leave the developer's workstation".
  See https://thoughtworks.github.io/talisman/
- Add .gitlab-ci.yml to build image from CI