Release of Alpine 3.10 Docker images

# .NET Core Alpine Docker images updated to Alpine 3.10

Docker images for .NET Core have been released that support Alpine 3.10. Included with this release, the floating tags for Alpine-based images have been moved from Alpine 3.9 to 3.10.

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

[dotnet/dotnet-docker#1418](https://github.com/dotnet/dotnet-docker/issues/1418)

## Details
* [dotnet/dotnet-docker#1415](https://github.com/dotnet/dotnet-docker/pull/1415)

If you made use of a floating tag for Alpine, such as `3.0-alpine`, `2.2-alpine`, and `2.1-alpine`, these have been changed to now reference Alpine 3.10 instead of Alpine 3.9. Be sure to pull this tag on your Docker machine to get the latest image.

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)

This is a completed release announcement, not an implementation task. Read dotnet/dotnet-docker#1415 for the image changes and dotnet/dotnet-docker#1418 for discussion; the listed Docker repositories are the release artifacts.

Written by the indexing model from the issue text.