Expose head/worker `volumes` and `volumeMounts` in values.yaml (for shared PVCs, etc.)

Summary

Expose generic volumes and volumeMounts at the head.* and worker.* values levels so operators can mount arbitrary Kubernetes volumes (PVCs, ConfigMaps, Secrets, emptyDir, hostPath, etc.) into the Ray head + worker containers without touching chart source.

The underlying RayService.spec.rayClusterConfig.headGroupSpec.template.spec and .workerGroupSpecs[].template.spec already accept any standard pod-spec volumes / volumeMounts — this is purely a matter of surfacing them through values.yaml.

Motivating use case

Sharing a large read-mostly dataset (or a model-weights cache) across the Ray head + all workers via a single RWX PVC. Concretely: mount an NFS/EFS/CephFS-backed PVC with accessModes: [ReadWriteMany] at /mnt/data on head + every worker so that:

• Actors don't each pull multi-GB datasets from remote storage on startup
• Multiple workers can read the same materialized dataset without duplication
• Checkpoints or intermediate output written by one worker are visible to others

This is a first-class pattern for Ray-based ML workflows (both training-adjacent and inference-cache scenarios). Right now, users hitting it fall back to helm post-renderers, kustomize patches, or vendoring the chart — all of which lose the "values file as the single source of truth" property that makes GitOps deployments legible.

Current chart surface (for context)

chart/values.yaml exposes for head + worker: containerEnv, resources, tolerations, readinessProbe, livenessProbe, and (worker) autoscaling replicas. The only mount surface today is orgCABundle (added in #16), which is hard-coded to CA-bundle-shape: a ConfigMap → an initContainer → a specific mount path. Great for its purpose, not generalizable.

Proposed shape

head:
  # existing fields...
  volumes: []           # standard pod-spec volumes list
  volumeMounts: []      # standard container-spec volumeMounts, applied to the ray-head container

worker:
  # existing fields...
  volumes: []
  volumeMounts: []

Example override a caller would use for the shared-PVC case:

head:
  volumes:
    - name: shared-data
      persistentVolumeClaim:
        claimName: ray-shared-data
  volumeMounts:
    - name: shared-data
      mountPath: /mnt/data

worker:
  volumes:
    - name: shared-data
      persistentVolumeClaim:
        claimName: ray-shared-data
  volumeMounts:
    - name: shared-data
      mountPath: /mnt/data

Implementation

Follow the exact pattern PR #16 established for orgCABundle — the surface + composition mechanics are already in the chart:

• Add named helpers in _helpers.tpl — e.g. nebari-rayserve.head.volumes, .head.volumeMounts, .worker.volumes, .worker.volumeMounts — that render the user-supplied lists (or empty when unset)
• In chart/templates/rayservice.yaml, concat the user-supplied lists with the orgCABundle equivalents so both features compose without one clobbering the other:

{{- $headVolumes := concat (.Values.head.volumes | default list) (fromYamlArray (include "nebari-rayserve.orgCABundle.volumes" .)) }}
{{- $headVolumeMounts := concat (.Values.head.volumeMounts | default list) (fromYamlArray (include "nebari-rayserve.orgCABundle.volumeMounts" .)) }}

Same pattern as the existing head.containerEnv + orgCABundle env-var concat in #16. When head.volumes / worker.volumes are empty (default), render should be byte-identical to today.

Alternatives considered

• Post-render / kustomize patches — works today but hides config from the ArgoCD Application spec; painful to maintain.
• Fork the chart — worst maintenance profile; diverges from upstream forever.
• Continue treating orgCABundle as the only supported mount surface — doesn't cover the common ML workload case of a shared dataset/weights PVC.

Acceptance

• head.volumes / head.volumeMounts and worker.volumes / worker.volumeMounts accepted as standard pod-spec / container-spec shapes
• Empty defaults ([]) produce byte-identical render vs. current output — verifiable via helm template diff
• Coexists with orgCABundle — enabling both simultaneously merges the volumes + mounts additively, not clobbering
• README section documenting the new fields with the shared-PVC example above

Start with chart/values.yaml, _helpers.tpl, and chart/templates/rayservice.yaml, following the existing orgCABundle and containerEnv composition from PR #16. Use helm template to verify empty defaults are byte-identical and that user volumes and mounts coexist with orgCABundle. Update the README with the new fields and shared-PVC example.