Thanks. I had to change it a little bit since the bundle install
as user discourse resulted in a permission denied error. But the following works:
apt-get update && apt-get install libmysqlclient-dev -y
git clone https://github.com/nlalonde/ruby-bbcode-to-md.git /tmp/ruby-bbcode-to-md
cd /tmp/ruby-bbcode-to-md
gem build ruby-bbcode-to-md.gemspec
gem install ruby-bbcode-to-md-0.0.13.gem
su - discourse
echo "gem 'mysql2'" >> /var/www/discourse/Gemfile
echo "gem 'ruby-bbcode-to-md', path: '/tmp/ruby-bbcode-to-md'" >> /var/www/discourse/Gemfile
exit
cd /var/www/discourse
bundle install --no-deployment --path vendor/bundle
su - discourse
cd /var/www/discourse
RAILS_ENV=production bundle exec ruby script/import_scripts/phpbb3.rb bbcode-to-md