Yeah it is still tricky. Flash makes it easier.
My process for custom container is to build locally on linux, make sure it works, then send up to cloud via docker hub
www.runpod.io/blog/introdu...
runpod.io
Introducing Flash: Run GPU workloads on Runpod Serverless: No Docker required
We've just released a way to run Serverless code without needing to build a Docker image: check it out.