I agree completely with Hooligan. Leaving ANY type of dog alone, outside for any amount of time is not a good idea. Especially for 8 hours and especially a Dobe.
The things I can think of going wrong can include the dog getting himself tangled up and not being able to get free. My friend once left her dog tied outside and he managed to hang himself by accident. You might think it's not possible but it can easily happen.
Someone could decide to steal him. This one is probably more common than you think too, and not just limited to purebreds, but they're especially at risk. Some dumbass sees a nice looking dog and no one watching him, consider that dog gone. There is an endless amount of idiots out there willing to do whatever they can to make a few dollars.
An animal could attack him. Not sure of your area, but if you're at all rural wolves, coyotes, bears, and (of course) other stray dogs could wander in your yard and attack. Since your dog would be tied up, he really would not be in the best position to defend himself and that fight could leave him seriously injured or worse.
The elements. Dobes are not outside dogs - not the way other breeds are, that love to live outdoors 24/7. Your dog needs access to warmth and shelter from any rain/wind/whatever.
8 hours is too long to leave him outdoors. Especially alone, with no one to check up on him. I can't tell you that you're not fit to own a dog, just that this would be an incredibly bad idea. I would never get a dog knowing that it would have to be alone for that amount of time (whether indoors or out) since it is simply not fair to the dog. If it's a new job (previously unforeseen situation) then I would do whatever it took to make sure that dog was being taken care of during those hours. Leaving him at doggy daycare, finding someone to come by to let him out a couple times, whatever.
I would strongly suggest reconsidering owning a dog at all. Like I said, I'm not saying you shouldn't own one, or are a bad owner or anything, just that not everyone has a lifestyle cut out for dogs. Think about it long and hard, and if you can't find a way to make sure the dog is happy and safe while you are at work, then you should ask yourself if you're really at the best point in your life to have one. Just my opinion.
Edit -- Just read your follow-up post and that sounds like a much better idea than leaving him outside. Sorry for ranting.
