Document why `DOTNET_RUNNING_IN_CONTAINER` is needed

This ENV is set in various `runtime-deps` and `runtime` images, and in various samples. It would be good to document why it is needed and which parts of the codebase honor that feature. That would help folks creating fully custom container deployments (not using our assets) to understand why setting this ENV does or does not matter.

https://github.com/dotnet/dotnet-docker/blob/7cf01d82858fcc3824574fb92580c4151954699a/src/runtime-deps/6.0/jammy/amd64/Dockerfile#L21