Do we know if dogs are asymptomatic transmiters of sars-cov-2?
It may be too early to tell.

Testing of dogs in Wuhan found no infected dogs
"487 dogs including 90 beagle dogs, 147 pet dogs and 250 street dogs during the outbreak of SARS‐CoV‐2 were also tested serological negative."
from https://onlinelibrary.wiley.com/doi/full/10.1111/tbed.13577

This article mentions deliberate infection of five dogs. on this very small sample they suggest that dogs are not likely to become carriers.

"Coronavirus can infect cats - dogs, not so much."
https://www.nature.com/articles/d41586-020-00984-8

found by searching "dog" on LitCovid at https://www.ncbi.nlm.nih.gov/research/coronavirus/