Thanks, but I’ve already tried the method described in the official 2014 guide you linked — and I can confirm that it no longer works for accessing private repositories via SSH keys during bootstrap.
I also tried the newer approach mentioned in one of the replies (mounting the SSH key into the container and configuring core.sshCommand in after_code), but unfortunately, that also failed in my case. Discourse still throws an SSH permission error when trying to clone the private plugin.
If there’s a currently working and reliable way to use a private repo with SSH in a production setup, I’d really appreciate more up-to-date guidance.