Include steps to only prompt for LUKS decryption passphrase once during boot

[hoalex]

First of all, thanks for the great guide! I successfully followed it and now have an encrypted ZFS RAID1 boot volume, including a working dropbear configuration.

One thing that is a bit inconvenient, however, is the fact that you need to enter your LUKS passphrase individually for each disk, even if the passphrase is identical among the disks. I found a guide on Stackexchange that shows how to use the decrypt_keyctl script in /etc/crypttab to only ask for the passphrase once (https://unix.stackexchange.com/a/392286).

The keyutils package came pre-installed on my Proxmox installation. Setting the keyscript=decrypt_keyctl option in /etc/crypttab, together with an identifier in the keyfile field, followed by update-initramfs -u -k all, did indeed have the desired effect of only having a single password prompt.

Let me know if you would consider accepting a PR that adds the above steps to your guide.

[mr-manuel]

I added this to the README, thanks!