Just add it to your app.yml in ~/var/discourse/containers with say nano app.yml and rebuild as per
## Plugins go here
## see https://meta.discourse.org/t/19157 for details
hooks:
after_code:
- exec:
cd: $home/plugins
cmd:
- git clone https://github.com/discourse/discourse-voting
(don’t remove the existing git clone commands, just add this to the list on its own line)