Zen's SSH public key is not configured in lizard's dropbear

Today we had to reboot Lizard and I couldn’t login into dropbear for unlocking the encrypted disk. We have to:

Verify/confirm that my key is indeed not configured in dropbear and included the initramfs.
Fix whatever is wrong in case the key is really not there.
Adjust the onboarding documentation in case a step is missing there.

Original created by @zen on 17051 (Redmine)

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