realVolumeSize mounts the NAMED VOLUME read-only into a throwaway alpine to du it
from a container view. No host path is involved — docker resolves the volume name
daemon-side — so it is the same safe class as the internal/backup/backup.go entry.
The gate was right to demand review; this diff is that review, on its own, because
burying an allowlist widening inside a feature commit is how an allowlist stops
meaning anything.
Tooling only: no Go change, no build, no deploy, no version bump.
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