I think the OP stated its likely a stray as dogs are dumped in her location.
We had a neighbors dog in my backyard. Luckily it was only chasing my chickens. We didn't shoot it only because we were not confident in the laws of livestock protection. We educated ourselves quick and returned the dog to the owner and let him know that next time it sets foot in our yard we will destroy it. And inform the police.
As much as the dog could be someone's family pet, once it kills it will likely do it again.
I think the OP should secure her chickens, but also find a way to remove the dog. Either SSS or having animal control come and collect it.
Stop promoting such. It is not accurate if people work with dogs. Allowing such to be followed is what promotes relations between dogs and livestock.