Fix sandbox hostname resolution for sub-containers

[phinze]

Sub-containers in a sandbox were getting auto-generated hostnames from containerd that didn't match anything in /etc/hosts or DNS. This went unnoticed because none of the existing addon processes (valkey, postgres, mysql) resolve their own hostname at startup. Erlang's EPMD does, and it dies immediately if it can't, which surfaced this during RabbitMQ addon work.

The sandbox architecture already shares network, IPC, and time namespaces between the pause container and sub-containers, but UTS (the namespace that holds the hostname) was missing. This meant each sub-container got a random containerd-generated hostname like 250a27c3c755 while /etc/hosts mapped a different name to 127.0.0.1.

The fix aligns with how Kubernetes pods handle this: set an explicit hostname on the pause container, share the UTS namespace to sub-containers, and make sure /etc/hosts agrees.

Test plan

• Blackbox test suite covers the general sandbox launch path (app sandboxes, addon sandboxes)
• Manually verified RabbitMQ addon sandbox starts successfully (previously crashed with epmd_error after ~6 seconds)

[coderabbitai]

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Reviewing files that changed from the base of the PR and between e8c0839 and fb4627b.

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• controllers/sandbox/sandbox_frozen_test.go

✅ Files skipped from review due to trivial changes (1)

• controllers/sandbox/sandbox_frozen_test.go

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📝 Walkthrough

Walkthrough

A helper sandboxHostname was added to derive a hostname by removing the "sandbox/" prefix from sandbox IDs. The sandbox controller now uses that trimmed hostname for host resolution and sets the pause container OCI spec hostname accordingly. Subcontainers are configured to join the UTS namespace so hostname isolation is consistent with the pause container.

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