Update default kernel for in-VM Docker networking

[sjmiller609]

Summary

• bump the default Hypeman kernel to ch-6.12.8-kernel-1.6-202603301
• add download and header URLs for the new kernel release
• keep the previous kernel versions available in the supported list

Testing

• go test ./lib/system/... was attempted in this checkout but is not runnable here because the repo is missing the embedded guest agent artifact and the init package expects Linux syscalls
• prior remote validation on deft confirmed this kernel enables Docker bridge/NAT support needed for in-VM Docker networking

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Note

Medium Risk
Changes the default Cloud Hypervisor kernel used for new instances, which can impact boot and networking behavior across the fleet. Added coverage is a manual, environment-gated integration test, so regressions may still slip through CI.

Overview
Updates Hypeman’s default Cloud Hypervisor kernel to ch-6.12.8-kernel-1.6-202603301, keeping prior kernel versions supported while adding the new kernel and header download URLs.

Adds a Linux-only, environment-gated integration test (docker_integration_linux_test.go) that boots a VM with an attached ext4 volume, installs and starts Docker using the volume for /var/lib/docker, and validates overlay2, container execution, and port publishing via an nginx container.

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