Not sure if this helps anyone but I was quite pleased with myself. To add an app to the cairo Dock. In thus case Darktable.
- Open the app
- Whilst hovering over the opened app icon in Cairo Dock right click and select create new launcher.
- This creates an icon in Cairo Dock which remains after you close the app.
- If you restart the computer the app boots up in Cairo Dock.
It maybe common knowledge but it also may help a beginner like myself who wants to know how to add the most commonly used apps to Cairo Dock.