Requirements:
Ubuntu 16.04.6 LTS
2 cpu cores
2 GB RAM
20 GB SSD

Install:
apt-get update
apt-get install mercurial default-jdk postgresql make

Modify postgres config:
vim /etc/postgresql/9.5/main/pg_hba.conf
host    all             all             127.0.0.1/32            trust

Restart postgres:
/etc/init.d/postgresql restart

Create role:
psql -h localhost -U postgres postgres -c 'CREATE ROLE administrator;ALTER ROLE administrator WITH SUPERUSER INHERIT CREATEROLE CREATEDB LOGIN REPLICATION BYPASSRLS;'

Create database:
psql -h localhost -U postgres postgres -c 'create database nabble owner administrator;'

Clone nabble:
hg clone https://hg.luan.software/nabble

Fix config
vim nabble/conf/Init.luan
replace me.nabble.com with server domain
configure MailHome.getSmtpServer with your smtp provider

Start:
cd nabble
./build.sh
./setup.sh
./serve.sh

Visit in browser:
http://domain/free-forum.html