in the book to build a fire how do we know from the end of the story that the dog has been somewhat domesticated?

The dog has enough instinct to survive in the wild, but at the end of the story it is specifically noted that he will make his way to the camp of the man's friends;

last sentence

"Then it turned and trotted up the trail in the direction of the camp it knew, where were the other food-providers and fire-providers."

last paragraph

"The dog sat facing him and waiting. The brief day drew to a close in a long, slow twilight. There were no signs of a fire to be made, and, besides, never in the dog's experience had it known a man to sit like that in the snow and make no fire. As the twilight drew on, its eager yearning for the fire mastered it, and with a great lifting and shifting of forefeet, it whined softly, then flattened its ears down in anticipation of being chidden by the man. But the man remained silent. Later, the dog whined loudly. And still later it crept close to the man and caught the scent of death. This made the animal bristle and back away. A little longer it delayed, howling under the stars that leaped and danced and shone brightly in the cold sky. Then it turned and trotted up the trail in the direction of the camp it knew, where were the other food-providers and fire-providers."