feat: allow configurable mode for secondary_boot_disk

[bbensky]

Summary

Parameterizes the hardcoded mode = "CONTAINER_IMAGE_CACHE" in the secondary_boot_disks dynamic block, exposing a new optional secondary_boot_disk_mode field on node_pools entries.

Why

GKE secondary boot disks support two distinct features that share the same API:

mode value	Use case
CONTAINER_IMAGE_CACHE	Preloads container images for faster pod start. Requires Image Streaming (gcfs_config { enabled = true }).
(mode omitted)	Preloads arbitrary data onto a node-local disk for pods to mount via hostPath.

The module currently only supports the first variant — mode is hardcoded since #1946. There is no way for a consumer to use the data-preload variant via this module today.

This change makes mode configurable per-pool, while preserving the existing default for backward compatibility.

Backward compatibility

Existing consumers using secondary_boot_disk continue to receive CONTAINER_IMAGE_CACHE automatically — the lookup() default matches the previous hardcoded value. No tfvars changes required for existing users.

Usage

For container image preloading (existing behavior, no change):

node_pools = [{
  name                = "my-pool"
  secondary_boot_disk = "projects/my-project/global/images/my-cached-image"
  # secondary_boot_disk_mode defaults to "CONTAINER_IMAGE_CACHE"
}]

For data preloading (new):

node_pools = [{
  name                     = "my-pool"
  secondary_boot_disk      = "projects/my-project/global/images/my-data-image"
  secondary_boot_disk_mode = null   # omits the mode field — data preload
}]

Files changed

• Source of truth: autogen/main/cluster.tf.tmpl and autogen/main/README.md
• Regenerated via make build and make docker_generate_docs (per CONTRIBUTING.md)

Tests

This is a small behavioral change with a backward-compatible default. I haven't added an integration test (would require a GCP project I can't easily provision); happy to add one if the maintainers prefer — pointers to the closest existing test welcome.

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[gemini-code-assist]

Code Review

This pull request introduces the secondary_boot_disk_mode parameter for GKE node pools across various modules and the main cluster configuration. The changes update the secondary_boot_disk block to dynamically look up the disk mode, defaulting to CONTAINER_IMAGE_CACHE while allowing users to override it or set it to null for data preloading. Documentation in the README files has been updated accordingly to reflect this new optional parameter. I have no feedback to provide as there were no review comments to assess.