Add Support for Multiple Docker Base Images on Official DockerHub

With testing for other base images right around the corner, https://github.com/AdoptOpenJDK/openjdk-tests/issues/1109, I wanted to make sure all correct base images made it into the testing pipeline. Thus allowing the new base images to get tested and pushed up to the official DockerHub repository, https://hub.docker.com/_/adoptopenjdk.

Currently the only base images in the official DockerHub repository are:

- Ubuntu (Bionic)
- Windows Server Core

The unofficial DockerHub repositories currently support:

- Ubuntu (Bionic)
- Alpine (3.10)
- Debian (Stretch)
- Windows Server Core (1803, 1809, LTSC 2016)
- Ubi-Minimal

It would be nice to start moving some of the base images in the `unofficial` DockerHub repositories to the official repository after testing has been completed.

In addition it would be nice to add support for the following additional base images:

- CentOS/ClefOS - #12 - https://hub.docker.com/_/clefos
- Windows Nano Server - https://hub.docker.com/_/microsoft-windows-nanoserver

Some other improvements that should be made while adding more base images/tags are:

- Update Debian base image from `stretch` to `buster` - #200
- Support Slim Builds in the official DockerHub repository
- Investigate `buildpack-deps` for base image pulls, especially the `curl` tags, https://hub.docker.com/_/buildpack-deps/

@dinogun @smlambert