Docs feedback: Unable to import SSH key for runner

https://docs.gitlab.com/ee/ci/ssh_keys/#ssh-keys-when-using-the-docker-executor

I see:

- echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add -

But this command does not work: not with ed25519 format, neither with rsa; not with tr -d, neither without it.

Error loading key "(stdin)": invalid format

I found similar issue #220880 (closed) without response.