Move ARM64 (linux/arm64) support from Alpha to Beta

[markmandel]

Summary

ARM64 support was introduced in v1.23.0 (May 2022) and has remained in Alpha for nearly three years. All Agones components produce ARM64 container images via Docker buildx, and pre-built SDK binaries are shipped for linux.arm64 and darwin.arm64. The missing piece is continuous integration on real ARM hardware — without it we cannot assert correctness or catch regressions.

This issue tracks the work needed to graduate ARM64 to Beta.

Criteria for Beta

Per the feature stage definitions:

• No known critical bugs.
• Automated testing covers the feature.
• Documentation is up to date.

Proposed CI Approach

Add a small GKE Standard cluster with ARM64 (T2A) nodes to the existing e2e test infrastructure, running a targeted subset of tests on every PR/merge — the same cadence as the existing x86 clusters.

Cluster spec (suggested):

• Machine type: t2a-standard-4 (Ampere Altra, ARM64)
• Node count: 3 nodes (smoke suite is small; full suite uses 10 — 3 is sufficient)
• Region: one of the existing e2e regions (e.g. us-central1 — T2A is available there)
• GKE release channel: RAPID (same as other e2e clusters)
• Provisioned via Terraform alongside the existing modules in build/terraform/e2e/

Test subset (smoke suite):

Rather than running the full e2e suite (~45 min per cluster), run a curated smoke suite that validates every major component is functional on ARM64:

Test	What it validates
TestCreateConnect	GameServer reaches Ready, SDK sidecar works, UDP connectivity
TestAllocatorWithSelectors	Allocator gRPC service end-to-end
TestRestAllocatorWithSelectors	Allocator HTTP/REST service end-to-end
TestGameServerShutdown	Graceful shutdown / SDK Shutdown() call
TestGameServerReserve	Reserve state transitions
TestFleetScaleUpAndDown (minimal)	Fleet controller and rolling updates
Ping test	ping component responds on ARM64

The subset can be specified via a -run regex flag in the e2e runner, keeping the ARM CI slot fast (target: under 20 minutes).

Tasks

• Add Terraform module for a GKE ARM64 (T2A) e2e cluster
• Add ARM64 entry to ci/e2e-test-cloudbuild.yaml child-build matrix
• Define and document the ARM64 smoke-test suite (regex or build tag)
• Confirm simple-game-server test image builds and runs correctly on ARM64
• Run the smoke suite consistently green across ≥ 2 weeks / several PRs
• Update site/content/en/docs/Installation/_index.md to change ARM64 stage from Alpha → Beta
• Add a blog/changelog entry noting the graduation

Notes

• T2A nodes are not available in all GCP regions; us-central1 is a confirmed availability zone.
• GKE Autopilot does not currently support ARM64 node pools, so Standard is the right choice here.
• The ARM64 smoke suite intentionally excludes controller-failure and HA tests — those remain x86-only until full Beta parity is established.

[galalmounir]

I'd like to work on this one, or at least part