I began packaging up this app with a custom Dockerfile (though the project does have its own dockerfile).
You can find the source at: https://source.infogulch.com/infogulch/promnesia-app though it is just barely running.
I have published the app package to my docker hub repo at infogulch/promnesia-app:v1.0.20210415-4
You may be well served by reviewing the app's own install docs from the README.md, or the users guide.
As for the cloudron integration, these are the steps that worked for me (I just got it working for the first time moments ago so...):
Install the server component with:
cloudron install -l subdomain --image infogulch/promnesia-app:v1.0.20210415-4
Then login to authenticate with proxyauth at the subdomain you choose to install it. The https://promnesia.my.example.com/status endpoint displays a short JSON status (the app is not designed to be interacted with from the web)
Install the extension.
Go to extension options and set the "Backend source" url to your domain, (e.g. https://promnesia.my.example.com)
It seems that logging into the app in the browser enables the extension to authenticate correctly. (This makes me want to critically review which extensions I have installed 😬.)