.NET Core `latest` Docker tags updated to 3.0

# .NET Core `latest` Docker tags updated to 3.0

The `latest` tags for the [dotnet/core](https://hub.docker.com/_/microsoft-dotnet-core) repos have been updated to reference [.NET Core 3.0](https://devblogs.microsoft.com/dotnet/announcing-net-core-3-0/).

## Discussion

You can share your thoughts on this change or see what others are saying at:

* [dotnet/dotnet-docker#1337](https://github.com/dotnet/dotnet-docker/issues/1337)

## Details

* [dotnet-docker#1334](https://github.com/dotnet/dotnet-docker/pull/1334)

The `latest` tags for the [dotnet/core](https://hub.docker.com/_/microsoft-dotnet-core) repos are always intended to reference the latest supported version of .NET Core. At this time, the latest supported version is .NET Core 3.0.

The following Docker repos have been updated:

* [dotnet/core/sdk](https://hub.docker.com/_/microsoft-dotnet-core-sdk)
* [dotnet/core/aspnet](https://hub.docker.com/_/microsoft-dotnet-core-aspnet)
* [dotnet/core/runtime](https://hub.docker.com/_/microsoft-dotnet-core-runtime)
* [dotnet/core/runtime-deps](https://hub.docker.com/_/microsoft-dotnet-core-runtime-deps)
* [dotnet/core/samples](https://hub.docker.com/_/microsoft-dotnet-core-samples)

It is recommended to use a more-specific tag than `latest` for anything other than casual development. Once you have an application that you believe will eventually go to production, you are recommended to use a tag like `3.0`. The [.NET Core Docker Samples](https://github.com/dotnet/dotnet-docker/tree/master/samples) contain examples of more specific usage such as the [.NET Core console sample](https://github.com/dotnet/dotnet-docker/blob/master/samples/dotnetapp/Dockerfile).