@Fosten Thanks for the report. It makes sense to me that we should try to support this.
I'm busy getting ready for ploneconf and might not be able to work on it for a little while. You could help move it forward by:
Give instructions on how to reproduce the problem (i.e. give an example of what needs to be added to the files generated by cookieplone in order to configure these addons to be fetched from source)
Cookieplone Dockerfiles do not currently develop add-ons from source. This is how I solved it.
In the frontend Dockerfile https://github.com/plone/cookieplone-templates/blob/6e8c38d31c345198262caf5fc03c0a1e748dba4b/sub/project_settings/%7B%7B%20cookiecutter.__folder_name%20%7D%7D/frontend/Dockerfile
Add these 2 lines to copy in and run mrs.developer:
In the backend Dockerfile https://github.com/plone/cookieplone-templates/blob/6e8c38d31c345198262caf5fc03c0a1e748dba4b/sub/project_settings/%7B%7B%20cookiecutter.__folder_name%20%7D%7D/backend/Dockerfile
mx sources are fetched but then deleted, so remove these 2 lines:
Another way to solve this might be to change the mx directory to ./sources instead of src