Yeah I think you'd have to probably use
https://hub.docker.com/r/pulumi/pulumi as a base and build on top of it to add .net 7. we recently added functionality for private images so it can be either public or private.
you can also join us at
#C048NVDH6DV for deployments-specific questions, there are folks there who have done some custom image building that might be able to help out.