My dog has a crusty skin ring with hair loss. Is it contagious?

The most common reason that we will see partial hair loss with a ring of crusting on the skin is a bacterial infection. Crusts can be a sign of a bacterial skin infection which is also called pyoderma. Here is an article on pyoderma.
https://www.vin.com/veterinarypartner/default.aspx?pid=19239&id=4951996

There are a number of treatment options for pyoderma. Sometimes we can resolve them with topical treatment (antibacterial shampoos, sprays or wipes), but sometimes they require oral or injectable antibiotics.

However, we still needs to look for the cause of the pyoderma. There are a number of reasons for skin problems in dogs. This article does an excellent job of describing them. You could be dealing with allergies (of various types) or external parasites (such as fleas or mites).
https://www.petcoach.co/article/why-is-my-dog-so-itchy/
This is also an excellent article on itching in dogs and the possible causes and treatments.
https://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2604

I would recommend having your veterinarian evaluate Stella to discuss your treatment options to resolve this skin problem.

Good luck! I hope that was helpful. Thanks for using PetCoach.