Add dockerfile for pre-build image

[fatmelon]

Add files for pre-build image and start services script.

• Navigate to the repo and build this image with:

  docker build . -f .\.github\containers\Build-Ubuntu\Dockerfile_prebuild -t <image-tag> 

• The image is now available for:

  docker pull mcr.microsoft.com/cosmosdb/ubuntu/documentdb-oss:20.04

• Base image is built from .github\containers\Build-Ubuntu\Dockerfile

[lichoil]

pls update the docker image in mcr using this dockerfile build after pr merged.