Build images using Docker CE 19.03 and BuildKit

[wborn]

Switching to BuildKit has many advantages:

• openHAB images are now based on the official Debian and Alpine Docker images instead of the multiarch images which:
  • fixes wrong architectures of non-amd64 images (#184)
  • improves security because these official images are reviewed by a dedicated team sponsored by Docker
  • allows for upgrading Alpine to 3.11 because of an issue the multiarch armhf image does not exist (https://github.com/multiarch/alpine/issues/30)
  • makes it possible to use the Debian "slim" image to further reduce the size of Debian images
• Multi architecture manifests are now generated automatically so we can remove the code for generating these and pushing them with manifest-tool
• Further reduces build times because images for different architectures are build in parallel

There is now a single Dockerfile per image instead of one for each architecture. BuildKit does not provide a way to also generate tags containing architectures so these tags will no longer be generated.

Furthermore the Travis configuration has been updated so it will also automatically sync the README.md to Docker Hub.

Fixes #184

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This change is 

[wborn]

Are you OK with these changes @cniweb?

[wborn]

I'll proceed with merging this. Feel free to make any comments on this afterwards @cniweb.

[bwosborne2]

Why was this breaking change made on a stable release and not even mentioned in the Release notes?? My Docker install script that worked fine in 2.5.3 was broken when I tried with 2,5,6, There were no breaking changes listed in the intervening Release Notes.