feat: enable multi-arch builds with ARM compatibility

[rhdedgar]

What does this PR do?

Enables lllama-stack multi-architecture builds for ARM. The result is an image index, that will point to the image associated with the calling user's architecture.

On Kubernetes for example, with these changes, images are still referred to in the same manner, but will resolve to the architecture that is relevant to the node where it is to be run.

                                              --> quay.io/<user>/distribution-starter/<version>-linux-arm64
quay.io/<user>/distribution-starter/<version>-|
                                              --> quay.io/<user>/distribution-starter/<version>-linux-amd64

In practice, referring to the usual docker.io/llamastack/distribution-starter:latest should resolve to the image with the requesting user's relevant architecture, without any additional configuration on the user's side.

Closes #406

Test Plan

Tested container builds on an amd64 architecture host, and deployed the starter distribution image via llama-stack-k8s-operator to an arm64 architecture OpenShift cluster. The deployment ran and the operator's e2e test suite completed as expected.

Arm64-specific build tests run and pass on GitHub.

[ashwinb]

Thanks for doing this! ❤️

[ashwinb]

this file is almost a duplicate of the other one. could we auto-generate them if we need them both or have some other way of unifying them?

[rhdedgar]

Hi, +1 to that. Just removing the recent additions of TARGETPLATFORM and BUILDPLATFORM was enough to make the Containerfile work for Podman as well. I've removed the duplicate Containerfile.podman addition.