Static Content Deployment: Symlink vs Copy Behavior on Read‑Only Filesystems

During a production deployment the setup:static-content:deploy command must place generated assets into pub/static while the application root is mounted read‑only. Magento can satisfy this requirement by creating symlinks or by copying files, but the documentation does not state which strategy is chosen or whether a configuration option forces one approach.

The deployment logs in var/log/system.log only report generic permission errors unless the command is run with -vvv, making it difficult to confirm the actual method used in automated pipelines.

Which configuration flag, if any, controls the symlink versus copy decision? Does the active deployment mode (production vs developer) influence the choice? How can the method be verified programmatically without enabling verbose logging?