Update ZeroTier version on Docker

This commit changes the Dockerfile ot use the multi-stage build and help
to get an automated build on Docker.

The idea of the multi-stage build is to use the already stable Debian
distribution channel to provide up-to-date versions of ZeroTier.

The benefit is that it would be possible to automate the image build,
either on Docker Hub, Travis or taking advantage of the [docker-library/official-images] infrastructure.

This changes follows the best-practices suggested by
[docker-library/official-images], such as using a High Availability GPG
keyserver, providing a default CMD, allowing "bash" on `docker run` and
others.

Given that both the builder `debian:stretch` and `alpine:latest` are
official messages and have [manifests], this means that this Dockerfile
is also multi-platform. This means that this same Dockerfile will
pick-up the correct Debian package according to the architecture of the
running system during build.

With this changes we could try to promote the image to be parte of
[docker-library/official-images], and take advantage of automated
publishing of multi-architecture images. Others would be able to use
`docker run zerotier` and download the latest version appropriate to
their system.

Related to #682

[docker-library/official-images]: https://github.com/docker-library/official-images
[manifests]: https://blog.docker.com/2017/09/docker-official-images-now-multi-platform/

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