Use Multi-Stage Build and add Windows 1809

[dduportal]

As https://github.com/docker-library/official-images/pull/5929 was merged, and documented here: https://github.com/docker-library/faq#multi-stage-builds, this PR introduces multi-stage build to support a new Windows 1809 image.

This PR fixes #44 , and is a follow-up of #49 (as the author is not responding).

By supporting the brand new multi-stage build, it also supersede #14 and closes #47.

It introduces the following changes (each change is an atomic commit for easier rollback if things go wrong during next release):

• Introduce Windows 1809 image
• Adapt the update*.sh scripts to the new multi-stage build process, with all the logic back to the script update.sh, and the 2 others scripts are just wrappers, kept to avoid breaking Traefik's CI for next release.
• Image "scratch" switch to multi-stage build process, simplifying the config (no need to split per arch.)
• Image "Alpine" templates had been updated to remove the $DOLLAR variable
• No more Traefik binaries stored 🎉
• Appveyor is now building for both Linux/amd64 and Windows
• Add timezone files in the image scratch

Closes #14 Closes #49

[dduportal]

Please note that Windows 1809 is not tested (yet) in AppVeyor, as the environment Windows Server 2019 is currently required. I've requested it to be available here (and on my fork), waiting for this.