Update docker files to use buildx to support multiple Windows OS

[jsturtevant]

Is your feature request related to a problem? Please describe.

Build upon #111 to convert the dockerfiles to use buildx so we can build versions for supported Windows OS's. This would solve #93 and #90

References:
pause image: https://github.com/kubernetes/kubernetes/blob/9a57dc20a98dcc7a6b2bd1323d4e27f84477cd1b/build/pause/Dockerfile_windows
test images: kubernetes/kubernetes#93889

Describe the solution you'd like
A clear and concise description of what you want to happen.

There would be the following images:

sigwindowstools/flannel:v0.12 (multi arch manifest that points to the latest of the following images)

sigwindowstools/flannel:v0.12-2019-{build_tag}

sigwindowstools/flannel:v0.12-1909-{build_tag}

sigwindowstools/flannel:v0.12-2004-{build_tag}

sigwindowstools/flannel:v0.13 (multi arch manifest that points to the latest of the following images)

sigwindowstools/flannel:v0.13-2019-{build_tag}

sigwindowstools/flannel:v0.13-1909-{build_tag}

sigwindowstools/flannel:v0.13-2004-{build_tag}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Setting up windows servers for each version. This requires to much maintenance and costs.

Additional context
Add any other context or screenshots about the feature request here.

[vitaliy-leschenko]

I did it! @jsturtevant please re-review last changes at #111

[vitaliy-leschenko]

I added kubeproxy images for all windows server versions also in #111.

[jsturtevant]

/close

[k8s-ci-robot]

@jsturtevant: Closing this issue.

In response to this:

/close

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