[WIP] Add Alpine SDK image

This PR isn't quite ready. Work items:

[x] Update URL format (to match other images)
[x] Add hash/checksum matching (to match other images)
[ ] Consider globalization invariant mode support for the SDK.
[x] Update tests appropriately to verify Alpine SDK image
[x] Update manifest.json
[x] Update readme

Sample I used for testing: https://github.com/dotnet/dotnet-docker-samples/tree/alpine/dotnetapp-prod

I got the image to work as expected but needed to remove invariant-mode support for the SDK. it seems like the SDK fails to run with invariant mode.

This is the error that I saw before removing globalization invariant mode / adding ICU:

Step 7/11 : RUN dotnet publish -c Release -o out
 ---> Running in 0f3ff19e0e0f
Microsoft (R) Build Engine version 15.6.22.57775 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 46.27 ms for /app/dotnetapp.csproj.
/usr/share/dotnet/sdk/2.2.0-preview1-007846/Microsoft.Common.CurrentVersion.targets(2051,5): error MSB3095: Invalid argument. SafeHandle cannot be null. [/app/dotnetapp.csproj]
/usr/share/dotnet/sdk/2.2.0-preview1-007846/Microsoft.Common.CurrentVersion.targets(2051,5): error MSB3095: Parameter name: pHandle [/app/dotnetapp.csproj]
The command '/bin/sh -c dotnet publish -c Release -o out' returned a non-zero code: 1

dotnet / dotnet-docker-nightly

[WIP] Add Alpine SDK image #508