BitBucket Pipelines: Upgrade Docker to version 19

The version of Docker used by Bitbucket pipelines is still on 18.09.1 which is nearly 9 months out of date. As of this post, version 19 was released 22 days ago and patched two days later.

As of July 2018, Docker has an established and predictable 6 month release cycle. It would be best if Bitbucket pipelines stayed in lock step with this. Alternatively, allow us to define the version of Docker we would like to use.

At a minimum, patch versions should be available ASAP for security and bug fixes. However major versions typically have much desired new features. Also, Docker has gotten much better at maintaining backwards compatibility with deprecation notices.

Please upgrade Docker to the current version for Bitbucket pipelines.