Merge dind mode template overrides for generated runner containers

[kiarashazarnia]

Summary

When using containerMode.type: dind, the gha-runner-scale-set chart auto-generates runner, dind, and init-dind-externals containers. Previously, user template overrides were inconsistent:

• runner overrides from template.spec.containers already worked
• dind overrides (e.g. resources) were ignored
• init-dind-externals overrides were ignored
• user template.spec.initContainers entries for generated dind / init-dind-externals could be appended as duplicates

Changes

• Merge user overrides into generated dind container from:
• Merge user overrides into generated init-dind-externals from:
• Filter generated dind / init-dind-externals out of appended user initContainers to avoid duplicates
• Use mergeOverwrite when combining dind overrides from both containers and initContainers
• Document customization paths in values.yaml
• Add regression tests for resource merge, default-empty resources, init-dind-externals merge, and duplicate filtering

Example

containerMode:
  type: dind

template:
  spec:
    initContainers:
    - name: init-dind-externals
      resources:
        requests:
          cpu: "50m"
          memory: "128Mi"
    containers:
    - name: runner
      resources:
        requests:
          cpu: "500m"
    - name: dind
      resources:
        requests:
          cpu: "100m"
          memory: "256Mi"
        limits:
          cpu: "200m"

Test plan

• go test ./charts/gha-runner-scale-set/... -v
• (gha) Validate Helm Charts CI passes

Notes

• No chart version bump (per CONTRIBUTING.md)
• No controller/runtime changes; ARC already passes the rendered pod template through to runner pods
• Backward compatible: if no override entries are provided, rendered pod specs are unchanged