ci: add arm64 container builds

[synackd]

Add 64-bit ARM container builds to GHCR.

This adds ARM64 containers to image-builder releases and nothing else. Testing and feedback are appreciated.

[alexlovelltroy]

It appears to build and push fine and I have confirmed that the arm version is included in the manifest.

https://github.com/OpenCHAMI/image-builder/actions/runs/18035983383

❯ docker buildx imagetools inspect ghcr.io/openchami/image-build-el9:test
Name:      ghcr.io/openchami/image-build-el9:test
MediaType: application/vnd.oci.image.index.v1+json
Digest:    sha256:fc83aef2d8fb7b1bd8b8bee332a06f886bf654d916fabdacaefea31b52c57f89

Manifests:
  Name:        ghcr.io/openchami/image-build-el9:test@sha256:1eee8fba6e0bdff61ead287015eff34ba35d3e07d894e1de4cbacb1c15aeb121
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    linux/amd64

  Name:        ghcr.io/openchami/image-build-el9:test@sha256:ec2b341ad7a47230ad05d4c173494c4a4be0b0c6a9582c80b784c11e69b2c27f
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    linux/arm64

  Name:        ghcr.io/openchami/image-build-el9:test@sha256:7fa52bf9b7459a005a6655f80c1f752b5fa5ef6525bb0aa35e5cb1f5ae75c7a0
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    unknown/unknown
  Annotations:
    vnd.docker.reference.digest: sha256:1eee8fba6e0bdff61ead287015eff34ba35d3e07d894e1de4cbacb1c15aeb121
    vnd.docker.reference.type:   attestation-manifest

  Name:        ghcr.io/openchami/image-build-el9:test@sha256:e5e3318afe2d8e26843a76e0e156f685b145b17361bcd803403aeb202382969b
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    unknown/unknown
  Annotations:
    vnd.docker.reference.digest: sha256:ec2b341ad7a47230ad05d4c173494c4a4be0b0c6a9582c80b784c11e69b2c27f
    vnd.docker.reference.type:   attestation-manifest

[alexlovelltroy]

LGTM

[synackd]

Thanks for testing! I'll remove the labels and we can merge this.