Update postgres
Update postgres

Changes:

- https://github.com/docker-library/postgres/commit/6274e4c: Merge pull request https://github.com/docker-library/postgres/pull/586 from piotrbrzuska/master
- https://github.com/docker-library/postgres/commit/966d1ba: Update 12+ to buster
- https://github.com/docker-library/postgres/commit/5992d8b: Update "Dockerfile-alpine.template" and apply "update.sh" (after verifying that 12 still builds and works properly without the added "-testing" repository)
- https://github.com/docker-library/postgres/commit/e3d200e: Add 12~beta1 into Travis config
- https://github.com/docker-library/postgres/commit/6f7881a: Add 12~beta1

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