Runs on the host: the two containers share a network namespace but not a
filesystem root, so this is the only place both trees are addressable.
A copy rather than a bindMounts entry. nixos-container refuses to start
when a bind source is missing, and the secret does not exist until
authelia's first boot has minted it. The registration token dodges that
with an activation script that pre-creates the file; that is unavailable
here, because tuwunel requires the secret to exist and be non-empty, so a
placeholder would satisfy the mount and then stop the homeserver.
Bounded wait then fail, never a silent skip: authelia's container can be
up while its generator is still minting.
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