Update Docker to 18.09

The latest base Bitrise Docker image has a bit outdated docker, it would great to have it updated to 18.09 to take advantage of buildkit builds.

I’ve create a pull request for it: https://github.com/bitrise-docker/bitrise-base/pull/266

Thank you, a review will come soon! :upside_down_face:

Closing as the GitHub PR was merged :tada:

Current version from system report (Android/Docker/Linux “latest” stack):

* docker: Docker version 19.03.1, build 74b1e89e8a