fix(azure): skip the redundant apt round trip when the image already has zram

[defangdevs]

Summary

• Issue Bake the runtime closure into an Azure image: bootstrap 3m47s, of which 797 MiB is one download #697's "bake it into the image" work already bakes the runtime nix
  closure AND linux-modules-extra (the zram kernel module) into the Azure
  Compute Gallery image (deploy/azure/bake-image.sh), and the bootstrap
  already skips its own nix installer when a box boots from that image
  (deploy/azure/agent-box.bicep, "nix already present (image-provided)").
• The apt half of that same logic was missing: every boot, image-provided or
  not, still ran apt-get update + apt-get install linux-modules-extra-...
  unconditionally, paying a network round trip and a redundant install to
  reconfirm a package the image already carries.
• This guards it the same way, on the same signal class the image actually
  provides: modinfo zram (does the kernel module exist on disk) rather than
  a dpkg database check, so it is true for an image-baked box and false for a
  stock-marketplace boot regardless of what else changed.

Context

Prompted by the comment on #697 asking to bake the remaining runtime
closure / apt work into the image. That issue's larger asks (a Community
Gallery, replication regions, rebuild cadence, storage cost) are explicitly
listed there as decisions for a human, not this bot, and none of them are
touched here. This is the one piece of "apt work" in that ask that was a
plain bug with a known fix and no decision attached: the image already does
the work, the bootstrap just wasn't told to trust it.

Test plan

• az bicep build --file deploy/azure/agent-box.bicep and committed the
  regenerated agent-box.json.
• python3 scripts/check_azure_template.py (with PyYAML on PYTHONPATH)
  — all 14 checks OK, including the JSON-drift check against a fresh
  build.
• Live verification against the existing gallery image (as perf(azure): use the allocated IP without settling #709 did for
  the static-IP change) is out of scope for this session — the change is
  a pure skip-guard with no new az calls, verifiable from the script
  logic and the existing bake-image.sh contents alone.

🤖 Generated with Claude Code

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

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Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 18e19248-00e7-44e2-97e5-0cdacc8a1c95

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/697-skip-apt-when-image-provided

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