Document scenario for SDK image with multiple runtime versions

There's been some interest in having an SDK image that contains multiple runtime versions. Examples: https://github.com/dotnet/dotnet-docker/issues/4160, https://github.com/dotnet/dotnet-docker/issues/2048. While this isn't something that we're likely to provide, we can document this scenario with guidance on how to configure a Dockerfile that satisfies this need.

For Linux, this would look something like this:

```Dockerfile
FROM mcr.microsoft.com/dotnet/sdk:7.0

COPY --from=mcr.microsoft.com/dotnet/aspnet:6.0 /usr/share/dotnet/shared /usr/share/dotnet/shared
```