Support BuildKit on Pipelines

A significant update for people who build docker containers pulling from resources secured by ssh (for example a private bitbucket repo).

It supports --ssh which acts as an ssh agent socket forwarder, giving access to whatever the host can access without having to do things like pass in an ssh key, mount it or other insecure workarounds.

https://github.com/docker/docker-ce/releases/tag/v18.09.0