Problem comes from this commit
Pinning to immediate previous commit did the trick.
In case anyone wants the workaround, define your plugin like this in hooks section of your discourse configuration yaml :
hooks:
after_code:
- exec:
cd: $home/plugins
cmd:
- git clone https://github.com/discourse/discourse-gamification && cd discourse-gamification && git checkout 6a399b4c220f31de62049504d27ff6e50611fbef