Alpine Docker images no longer include the tzdata package in 3.0

# Alpine Docker images no longer include the tzdata package in 3.0
Beginning with the 3.0 release, the `tzdata` package has been removed from all Alpine Docker images.

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

dotnet/dotnet-docker#1366

## Details
* https://github.com/dotnet/dotnet-docker/pull/1312

This package was removed because it wasn't required for the main set of scenarios and to have consistency between the runtime-deps images across all Linux distros. This package is still installed in the 2.x images for Alpine so this will be something to be aware of when migrating from 2.x to 3.0.

You may be impacted by this if you use any of the Alpine images and your code has a dependency on the `tzdata` package, such as using the `System.TimeZoneInfo` API. In that case, you'll need to update your Dockerfile to explicitly install the package:
```
RUN apk add --no-cache tzdata
```

Read the announcement details and the referenced dotnet/dotnet-docker pull request 1312, then review the related discussion in dotnet/dotnet-docker#1366. The documentation is complete when the Alpine 3.0 tzdata removal, affected TimeZoneInfo scenarios, and explicit apk installation guidance are clearly communicated.

Written by the indexing model from the issue text.