I just noticed the commit hash for docker_manager is different.
I ran
./launcher enter app
root@tarck-app:/var/www/discourse/plugins/docker_manager# git log --name-status HEAD^..HEAD
commit 61175ada7a1993647878c53320e9c6ffb1076dd2
So the repo is up to date but Discourse is showing an older hash (from Tuesday 18th Sep). I might try rebuild again and see what happens.