Okay, I managed to start postgresql service, did database backup and entered to the database:
# chown -R postgres /shared/postgres_data
# service postgresql start
* Starting PostgreSQL 10 database server [ OK ]
# psql
psql: FATAL: role "root" does not exist
# su - discourse
$ pg_dump -xOf ~/discourse-backup.sql -d discourse -n public
$ exit
# mv /home/discourse/discourse-backup.sql /shared/discourse-backup-20200227-1.sql
# su - postgres
$ psql discourse
psql (10.4 (Ubuntu 10.4-2.pgdg16.04+1))
Type "help" for help.
discourse=#
Now, what can I do to recover my installation?