Supporting new Alpine versions (for containers)

# Supporting new Alpine versions (for containers)

[Alpine 3.9](https://twitter.com/alpinelinux/status/1090416212822110214) was recently announced. Congratulations, Alpine team! On the .NET team, we are committed to supporting new Alpine versions quickly, and also need to define a policy for older versions. This policy only applies for the container images that we publish.

## Discussion
Please share your thoughts on this change and see what others are saying at:

* [dotnet-docker #933](https://github.com/dotnet/dotnet-docker/issues/933)

## Policy

We intend to establish the following policy for supporting Alpine with .NET Core container images:

* Support new Alpine versions within 60 days of release (hopefully much less).
* Support n-1 Alpine versions for 3 months after we support a new Alpine version.

Note: The [.NET Core support policy](https://github.com/dotnet/core/blob/master/os-lifecycle-policy.md) may be broader. This policy is specific to the container images we produce.

## Plans

We're in the process of [fixing issues required to support Alpine 3.9](https://github.com/dotnet/dotnet-docker/issues/921). We expect that these issues will be resolved in February and that we'll be able to provide support for Alpine 3.9 in March.

We will produce Alpine 3.9 images for .NET Core 2.1, 2.2 and 3.0. Assuming we publish 3.9 images in March, we'll stop producing and supporting Alpine 3.7 and 3.8 images after June (2019).

## Using Alpine-based .NET Core images

We produce a variety of Alpine-based images. You can either depend on a specific Alpine version, like with `microsoft/dotnet:2.2-sdk-alpine3.8` or opt to use the latest Alpine image we have to offer, like with `microsoft/dotnet:2.2-sdk-alpine`. Either are good choices. It is just a question of whether you want to make a conscious decision to move to later Alpine releases.

## Support for ARM32?

We produce Alpine images for x64 (and ARM64, shortly). The [Alpine 3.9 announcement](https://twitter.com/alpinelinux/status/1090416212822110214) made mention of support for ARM32v7. We would like to enable .NET Core [Alpine-based images on ARM32](https://github.com/gliderlabs/docker-alpine/issues/463#issuecomment-460530395). We are patiently waiting for that to happen. If that's important to you, you should comment on that thread.