I was thinking along the same lines, and also struggling with Dor. I was wondering if it might represent the Old Germanic Thor which occurs in some English surnames in various forms; Thirkettle, Torode, Thorburn, etc. I believe the early medieval German form of Thor was Donar, which makes this explanation less likely. Anyway I don't know of any German surnames that incorporate the name Thor, however spelt. Maybe someone with a knowledge of German can clarify.