Add multi-arch container image support

[mrIncompetent]

Summary

Adds multi-arch container image support for linux/amd64, linux/arm64, linux/s390x, and linux/ppc64le.

Closes #372

Changes

• CI build job: Replaced the single-platform docker build with a matrix job that builds all four architectures using Docker Buildx and QEMU
• CI push job: Pushes per-arch images and assembles them into a manifest list via docker manifest create
• Deployment configs: Removed kubernetes.io/arch constraints from Helm values, kustomize manager config, and OLM bundle CSV — with multi-arch images those aren't needed anymore (Or should those be kept for backwards compat?). Only kubernetes.io/os: linux is retained, following the pattern used by cert-manager and ingress-nginx.

Build time

Cross-compilation via QEMU takes up to ~18 minutes for the non-amd64 architectures (amd64 builds in ~1.5 minutes natively). The image builds currently block E2E tests since GitHub Actions doesn't support depending on a single matrix entry. This could be changed by splitting into separate build-amd64 and build-cross jobs, but the added duplication isn't worth the complexity IMHO.

Testing

Validated end-to-end on our fork by temporarily swapping quay.io/backube/snapscheduler for ghcr.io/niledatabase/snapscheduler to test the full build + push pipeline:

• Master push (publishes latest manifest): https://github.com/niledatabase/snapscheduler/actions/runs/23814407119
• Tag push v99.0.0-test (publishes version-tagged manifest): https://github.com/niledatabase/snapscheduler/actions/runs/23815806152

Both runs show all four architectures building, E2E passing on amd64, and the manifest list pushed successfully.

docker manifest inspect ghcr.io/niledatabase/snapscheduler:latest

{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
   "manifests": [
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3233,
         "digest": "sha256:cbc75c031b313175a3db5ee86c4d297ba54a02ba910a9a2c8b3115294661df3f",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3233,
         "digest": "sha256:75f275e0fa55093e236f8ecbf88ce704a619c9d06793e0a316d9c5803a9fa803",
         "platform": {
            "architecture": "arm64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3233,
         "digest": "sha256:90803657434b6e5c7e376160ba2b52527148851a50ddc52e3974b10328e39c1b",
         "platform": {
            "architecture": "ppc64le",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3233,
         "digest": "sha256:227f0139b686a8dabd2ed77b80e109ae123a41102dca6d481922431bfeef0b08",
         "platform": {
            "architecture": "s390x",
            "os": "linux"
         }
      }
   ]
}

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This PR was authored with Claude Code.

[mrIncompetent]

Ping @JohnStrunk

[mrIncompetent]

Ping @JohnStrunk :)