Is your dog doesn’t bark? What’s the age of your dog? Around one, then no problem it is a puppy which had not learn barking. Some dogs start barking at strangers after one year and some dogs which is friendlier breed will start to bark at the age 2 or more than that.
Did you buy your dog for guarding purpose or as a pet dog? Is that age is above two? If your dog is guard/watch dog and you brought it for that purpose then it is better to train them using professional trainer.
If it is only the pet dog, then leave your worries. Your dog will be well-socialized and you can train them.
Say a word like bark anything of your choice. If anyone knocks the door, say him “bark”, give immediate treat for his first sound or bark made by him. He soon understands the word “bark”. Once when he learn, you can move to next place or location in and around your house. Next you can go out and knock the door and command him to bark. Then make your family members to do that. Don’t forget him to praise him or giving small treat or any toys or massage him every time. Next time your dog should not know who is going outside. Repeat the same thing.
Do not do this for a long time; your dog will become bored or tired. Don’t encourage him for more barking. Only 1 or 2 barks are enough at starting. Also try him to be quiet using the word “quiet” or something similar. Give treat or praise him when he stops barking.