Explore multi-arch images

[psolymos]

This post and this PR.

The faas-cli help publish says:

Builds and pushes multi-arch OpenFaaS container images using Docker buildx.
Most users will want faas-cli build or faas-cli up for development and testing.
This command is designed to make releasing and publishing multi-arch container
images easier.

A stack.yaml file is required, and any images that are built will not be
available in the local Docker library. This is due to technical constraints in
Docker and buildx. You must use a multi-arch template to use this command with
correctly configured TARGETPLATFORM and BUILDPLATFORM arguments.