I am Linux system administrator with some experience with IPv6 configuration, both on the network part and servers part (DNS, mail, web, etc). I had no experience with SIIT-DC, never heard of it, not until a few days ago, when I first saw this project.
However, I try to read those documents about jool setup, and also attempt small virtual network on virtualbox environment with 3 Linux machines, and try to register to hetzner cloud, create a linux server using debian, and learn about the target environment. From my limited understanding at the moment, with hetzner cloud that has only 1 ipv4, you can't configure SIIT-DC. You need that one ipv4 address for gateway address of ipv4 node to contact with ipv6 node. You also need a set ipv4 address for mapping to ipv6 node.
To setup hetzner cloud server as SIIT-DC node, assume that we have all ipv4 address, ipv6 prefix that we need.
Assume the server is debian 11, then we need to install two additional package for jool,
$ sudo apt-get install jool-dkms jool-tools
since the 'main' interface for server is already configured by hetzner cloud script, for both ipv4 and ipv6 we can skip that.
so the rest would be
$ sudo sysctl -w net.ipv4.conf.all.forwarding=1
$ sudo sysctl -w net.ipv6.conf.all.forwarding=1
and then the jool part
$ sudo modprobe jool_siit
$ sudo jool_siit instance add default --netfilter --pool6 $IPv6_PREFIX
and then the emat part
$ sudo jool_siit -i default emat add ...
we are run out of space here.