First up, you will need to check on the status of a diabetes assist dog. It might have an official position, it might not. If it has, go with Snow's Answer. Just make sure you know your and the dogs rights, if any apply.
If dogs like a diabetes assist dog don't have official status. This might be reasonable approach to getting a (your) dog on the work floor. Any dog for that matter.
Talk about it with your colleagues. That will give you an idea if people are for or against it. Mind that it takes only one person with an allergy to keep dogs out of the office. If it is not a working dog.
If everyone on the work floor is at least neutral on the subject, go talk about it with your boss. This order does not matter much, you need both on your side.
I'd say you have about 1 in 5 chance for a dog in the office. Probably less. Offices that have dogs, or allow them, do make the news.
Work less hours and close to work. So you can own and properly care for a dog. Without the need to have him at work.
Find an employer that allows dogs in the office.
Work from home or create your own business.
*I am not up to date on the German laws on this, go it's a generic "get people on your side" thing.