Docs on building custom image

[Oudwins]

You used to have a Dockerfile.cuda and Dockerfile.cpu which was what I was using to build custom faster-whisper-servers. But those got removed in favor of the new docker compose files. But docker compose files use the prebuilt image. Could we have some docs on how to run a server with a custom image (i.e if I want to change some of the code on the server and run that in a container rather than the prebuilt image)