Congratulations on the new job.
Even though you may not feel stressed or nervous about your new job there is something in your brain which has these feelings and that is why they are coming out in your dreams.
It is completley natural to have these feelings when starting out on something new and Im sure they will stop as soon as you start your new job, unfortunatley there is nothing you can really do to stop them before hand.
Sometimes when I have been stressed in previous jobs I have started to have dreams that I am at work and I will act out my dreams! I once had a job in a smoothie bar and I HATED it, I would wake myself up 4-5 times a night acting out my dreams looking in my drawers for fruit for the smoothies LOL! Once I also worked in a large department store but was only on a temporary contract, I knew this contract was coming to an end soon and was stressed about finding a new job... I again woke myself up many times in the night folding my quilt up thinking it was an item of clothing that I was packing for a customer! LMAO.
Now I am in a job that I love which is totally stress free, I have been here over a year and have NEVER had these dreams again.
I think somewhere in your mind you are obviously stressed, anxious or worried about starting your new job, or this could even be excitement! either way it is coming out in your dreams... Maybe watching a film or reading a book before bed might help? I find if I do this before I go to sleep I usually dream about the film or book I have just read/watched.
Dont worry it wont last forever