Add image publishing and Kubernetes sample

[gjkim42]

Summary

• Add a Dockerfile and make image target for building the Agora server image.
• Add CI coverage for the image build and a Release workflow that publishes ghcr.io/<owner>/<repo>:main on pushes to main.
• Add a Kubernetes sample manifest with a namespace, PVC, deployment, and service.
• Update the README to document Kubernetes usage instead of a local container run flow.

Verification

• Parsed the CI workflow, Release workflow, and Kubernetes sample as YAML.
• Confirmed there are no remaining QEMU or multi-platform Buildx references.
• Ran git diff --check.
• Ran make -n image IMAGE=agora:ci.
• Ran GOCACHE=/private/tmp/agora-make-image-gocache make verify.

Notes

• The image build is intentionally single-platform; QEMU and explicit multi-platform Buildx arguments were removed.
• kubectl apply --dry-run=client could not be completed locally because this kubectl attempted API discovery and cluster credential access, so Kubernetes validation was limited to local YAML parsing.