*: build multi-arch docker images linux/amd64,linux/arm64

[danbf]

Problem

current nsq images don't support arm64 architecture. that means we can't run them on AWS Graviton's or Apple M1's natively.

Solution

use buildx to build a multi-arch with linux/amd64,linux/arm64 platform support.

the biggest change is we have to go from separate build and push steps into a single build and push step. this is because the local docker doesn't support building locally multi-arch images.

Notes

based on https://www.docker.com/blog/multi-arch-images/ and https://www.docker.com/blog/multi-arch-build-what-about-circleci/
our upstreams, both golang:latest and alpine:3.10`` support these architectures and more.

addresses: #1306

[mreiferson]

I'll give this a test run, seems like a good time for a stable release ;)