Proposal: support multi-arch functions

Current Behaviour

As for now if I want to have a function, which runs on amd64, and ppc64le, I have to do following:

• Create a new function

$ faas-cli new function --lang golang-http

• Change function.yml to have 2 functions for 2 architectures:

version: 1.0
provider:
  name: openfaas
  gateway: http://gateway:31112
functions:
  function-amd64:
    lang: golang-http
    handler: ./function
    image: id/function:latest-amd64
  function-ppc64le:
    lang: golang-http-ppc64le
    handler: ./function
    image: id/function:latest-ppc64le
  function:
    image: id/function:latest

• Build 2 functions:

$ faas-cli build -f ./function.yml

• Push them to docker
• Create manifest

$ docker manifest create id/function:latest id/function:latest-amd64 id/function:latest-ppc64le

• Push manifest

$ docker manifest push id/function:latest

• deploy the function

$ faas-cli deploy -f ./function.yml --filter function

Expected Behaviour

Something like:

faas-cli up -f ./function.yml -a amd64,pcc64le

Context

I just want to build functions on my mac, test them on mac and on intel, and deploy them into production anywhere.

Your Environment

• FaaS-CLI version ( Full output from: faas-cli version ):
  0.8.21
• Docker version ( Full output from: docker version ):
  18.06.0-ce
• Are you using Docker Swarm (FaaS-swarm ) or Kubernetes (FaaS-netes)?
  Kubernetes
• Operating System and version (e.g. Linux, Windows, MacOS):
  MacOS, Linux

Start by tracing the faas-cli up, build, and deploy commands and how they read function.yml. Compare the current separate-architecture workflow with the proposed -a amd64,ppc64le option. Done means a multi-architecture function can be built, pushed, and deployed through the CLI without manually creating Docker manifests.