Document the C++ cloud build image (Debian Bookworm)

[btshrewsbury-viam]

Feature

C++ cloud build image (announced 2026-04-03, "C++ Cloud Build & Module Generator CLI"). C++ modules can be compiled through Viam's cloud build infrastructure using a standardized Debian Bookworm Docker image, alongside C++ module scaffolding via the CLI generator.

Current docs coverage

Partial. The C++ module generator is documented (docs/cli/reference.md --language cpp, docs/cli/build-and-deploy-modules.md), and cloud build is documented generally (docs/build-modules/overview.md, platform tags). Missing: the specific detail that C++ cloud builds run on a standardized Debian Bookworm image (all bookworm hits in the docs are about viam-agent/OS support, unrelated to the C++ build image).

Verified state / source PRs

• viamrobotics/viam-cpp-sdk#548 — "Introduce lockfile for reproducible cloud builds" (merged)
• viamrobotics/viam-cpp-sdk#616 — cloud build linkage fix (merged)

Proposed docs work

• Type (Diátaxis): reference note (small).
• Placement: add the build-image detail to the existing cloud build / module build docs (docs/build-modules/ cloud build section). Additive — a sentence/table row, no new page.
• Draft learning objectives — users will be able to:
  1. [Understand] Identify which base image (Debian Bookworm) Viam cloud build uses to compile C++ modules, so they can match their local build environment and dependencies.

Target PR: small additive edit to the cloud build docs.

[btshrewsbury-viam]

Update from drafting (PR #5213): the specific Debian Bookworm codename could not be confirmed from source and the evidence conflicts — viam-cpp-sdk#548 generated its lockfile from a bookworm job, but viam-cpp-sdk#616 (merged after the announcement) fixes the bullseye cloud build, and the SDK ships for both. The only file literally named bookworm (rdk/cli/module_generate/cpp-gen/Dockerfile.debian.bookworm) is the dev container for the stub-generator tool, not the module cloud-build image. Per viamrobotics/build-action, Linux cloud builds use the same auto-updating Debian layer that builds the RDK. PR #5213 therefore documents the verifiable fact ("Debian-based, same layer as the RDK build") and directs readers to install deps with apt-get in the module setup step, rather than asserting an unverified codename. If the team has a definitive, non-stale answer on the exact codename, the note can be tightened.