Add support for linux/arm64

[magiconair]

We are building a cluster running both linux/amd64 and linux/arm64 nodes. We run the coredns-nodecache on the nodes and need to support linux/arm64. I'll fork the repo and try to create a PR but if you have support for this somewhere already then I'd be happy to take it.

[yannh]

cc @elblivion I have positive experience using goreleaser, not sure if that's something we would be interested in adding here?

[magiconair]

goreleaser would only create the different coredns binaries AFAIR but we also need base images for the amd64 and arm64 architecture and create a multi-architecture manifest so that you can just pull by name and the client will get the correct image.

[hoangminhtu-dh]

My understanding is we'd need to tweak

coredns-nodecache/Makefile

Line 2 in c637fac

docker build -t contentful-labs/coredns-nodecache .

to use the new buildx. Example: https://github.com/kubernetes/node-problem-detector/blob/9b2d0be9504e7b7c95f053af51bcad520a5b46de/Makefile#L272 (PR: https://github.com/kubernetes/node-problem-detector/pull/684/files).