We’ve just hit this issue too. The launcher fails on CentOS 7 for many reasons, including complaining that the storage system devicemapper is unsupported. The image builder also craps out because it does not support selinux.
After reviewing how this launcher works I think the best course of action is to throw it under a bus. It is a fractal of poor decisions. It makes a joke of running containers anywhere the Docker daemon runs. It should be made clear that Discourse is supported only on “Ubuntu”. The Docker support is incidental. Don’t expect to run this on ECS, CoreOS, Kubernetes etc.