Well I just do happen to have a motormans certificate on diesel
engines, so perhaps I'm competent to comment.
Depending on the diesel, you don't need t start it all that often. I start mine here in DK once a month or so.
The big issue is you must run it until it is hot. Which means running it under load. Idling won't do it. Run it while moored - fine. Just make sure you get the oil and the engine hot.
If you DON'T have a turbocharger, drop the engine back to idle, once it is hot. let it idle for a moment and shut it dow.
If you do have a turbo, you will need to let the turbo cool down before shutting off the engine.
Best thing you can do for your engine is change the oil and the filter every year (even if you haven't used it much)