My main question is: why are you doing that? 
Things to check:
-
Does the ember-cli process have access to the plugins directory?
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What does your shared volume include? The whole Discourse directory? In particular, I’m thinking of
public/assetsandapp/assets/javascripts/discourse/dist -
Where are your HTTP requests going? Ember-cli’s proxy? Or straight to the Rails app? (either can work, but knowing which way round you’re doing it may help debug)