I'm no expert but looking at their install script for install.sh, most of it can be achieved without needing docker-compose anyway.

My first step looking at an app with a view to packaging it is to search if the app has a Dockerfile, then to modify that for Cloudron.

EDIT : looking at the content of their install.sh, if you are working from cloudron base image 4.0xxx, all of it is redundant apart from the last line
sudo npm install -g create-erxes-app -y
Maybe your Dockerfile that you are constructing should use that and then further steps as advised in step 4.
Plus setting up DBs in your Dockerfile.

EDIT 2 :
if I was attempting to package this, I would work from their steps in https://docs.erxes.io/docs/deployment/installation/installation-ubuntu
The only things they use docker for are to work with mongo, redis and rabbitmq.
The rest of their steps (8 onwards) would need implementing (if needed) in your Dockerfile, plus setting up mongo, redis, rabbitmq in your Dockerfile first.