Alpine 3.9 has been released and the microsoft/dotnet alpine images should be updated to based on it.

The following is the proposal.

1. 3.0 will only have 3.9 images of Alpine.
2. Add 3.9 Alpine images for 2.1 and 2.2. The floating alpine tag will be updated to reference 3.9. Keep the older alpine images for a period (3 months) before being deprecated.

Related issues:
https://github.com/dotnet/corefx/issues/33870
dotnet/corefx#34789