I found a way.
1. First, need do download and install a debian archive keyring package that contains a GPG key for Debian 8 Jessie:
wget https://archive.debian.org/debian/pool/main/d/debian-archive-keyring/debian-archive-keyring_2017.5+deb9u1_all.deb -O debian-archive-keyring.deb ar x debian-archive-keyring.deb xzcat data.tar.xz | tar -C / -xf -2. Then, edit the file /usr/www/cgi-bin/deb-mirrors.txt and add a line with a archive repo mirror (since Debian 8 Jessie is unavailable on default mirrors):
http://archive.debian.org/debian ARCHIVE
Or follow the installation via web UI and put http://archive.debian.org/debian on Debian mirror field.
3. After that, start the install process via web UI using the ARCHIVE Debian mirror from the list.
Troubleshooting
- If you got the error:
E: Execution failed: No such file or directory W: Couldn't validate Release!
Just ignore!
- At the end of installation process, if you get the error:
Setting the root password to be the same as Alt-F web admin password... /bin/bash: ***: command not found root:***
Probably because of special chars on your password. Run the command:
Replacing <your_password> with your web admin password.