I have created a Azure Container Registry, and uploaded a custom ACI to the registry - no problem - everything works as intended. I have tried creating an container instance
I had the same problems for the last couple of weeks and I've finally found a solution. You should add your azure registry server name in front of the image name. So following your example change:
.WithImage("mytestimage/latest")
To:
.WithImage("mytestreg.azurecr.io/mytestimage:latest")
At least that did it for me, I hope it helps someone else.