Add multi-arch support, with fallback to current behaviour

[alexellis]

Task

Add multi-arch support, with fallback to current behaviour

Update the existing Dockerfiles as per: https://github.com/alexellis/multiarch-templates/blob/master/template/go-multiarch/Dockerfile

Testing instructions

Test as per: https://github.com/alexellis/multiarch-templates

And make sure that faas-cli up still works without any multi-arch considerations.

[utsavanand2]

I would love to take a go at this

[alexellis]

Please do. You should just need to copy and paste from the example above.

[utsavanand2]

@alexellis PR #48 addresses this issue