Support armv7/8 and arm64

[vchrisb]

It would be great to have support and docker images for armv7/8 and arm64.

I've seen #801 but godep now supports and builds armv7 and arm64. (golang/dep#2102)

[vchrisb]

I was able to successfully build external-dns with GOOS=linux GOARCH=arm64 and run it on an arm64 kubernetes cluster with google DNS.
So I guess it is "only" necessary to modify the Makefile for build and also modify the Dockerimage to support multi arch.

Here is my test image: https://cloud.docker.com/repository/docker/vchrisb/external-dns

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