You need to enter the Docker container:
./launcher enter app
Then install the mysql2 gem…
apt-get update && apt-get install -y nano libmysqlclient-dev
cd /var/www/discourse
echo "gem 'mysql2'" >> Gemfile
su discourse -c 'bundle install --no-deployment --without test --without development --path vendor/bundle'
… configure the import script
nano script/import_scripts/phorum.rb
and execute the importer
su discourse -c 'bundle exec ruby script/import_scripts/phorum.rb'