rstudio-workbench: set imagePullPolicy on launcher init containers

[stevenolen]

Fixes #949

Summary

The .Job.initContainers loop in files/job.tpl emitted no imagePullPolicy, so Kubernetes defaulted to IfNotPresent for any non-:latest tag. With launcher-positron-preview-init-container-enabled, the Positron preview init image (ghcr.io/posit-dev/workbench-positron-init-preview:daily) was pulled once per node and then reused indefinitely, so different nodes served different (and stale) preview builds.

Changes

• files/job.tpl: for launcher-provided init containers:
  • images ending in :daily or :latest get imagePullPolicy: Always;
  • otherwise, launcher.templateValues.pod.initContainerImagePullPolicy is emitted if set;
  • if empty (the default), nothing is emitted, which keeps today's behaviour.
• values.yaml: new launcher.templateValues.pod.initContainerImagePullPolicy: "".
• Chart version 0.22.4, NEWS entry, regenerated README.

The rule keys off the image tag rather than off whether preview is enabled: the template only sees the resulting image string, and a chart-wide switch would force Always onto pinned images for no benefit.

# Version: in job.tpl is unchanged; it tracks the upstream launcher template version, and chart-side edits to job.tpl have not bumped it.

Testing

• make lint passes; just test rstudio-workbench passes (173 tests).
• Rendered all lint/ scenarios on main and on this branch: the only differences are the new job.tpl lines, the new key in the template-values JSON, and the resulting checksum/config-general annotations.
• Rendered job.tpl against sample init containers (...-preview:daily, foo:latest, posit/workbench-session-init:2026.09.0):
  • value empty: Always, Always, unset
  • value IfNotPresent: Always, Always, IfNotPresent
  • value Never: Always, Always, Never

Out of scope

This covers launcher.useTemplates only. Without templates, the Kubernetes launcher builds the pod itself and Workbench's launcher InitContainer has no pull-policy field, so a complete fix needs a Workbench change (see the issue).