Add support for Alpine 3.12

[rbhanda]

This issue is created to track progress for the work to enable Alpine 3.12 support for .NET Core 3.1 and .NET 5.

Product and Test updates

• [All teams] Do a compatibility check for the product with the new OS
• [CoreCLR/CoreFX] Runtime / RID Support update update Ubuntu and Alpine RID runtime#37031
• [Infra] Ensure that the correct Docker images used by teams are built in the dotnet-buildtools-prereqs (if applicable) https://github.com/dotnet/core-eng/issues/9965
• [Infra] Replace docker tags used in builds across repos and branches
• [CLI team] Update CI legs to latest updated Dockerfile
• [All teams] Update test infrastructure to support the new version
• [Acquisition] Add support within nightly repos
• [Acquisition] Deploy to master (GA) during next patch release cycle

Release team:

• Notify .NET Core teams regarding new OS release
• Add feed for new OS release to packages.microsoft.com (if applicable)
• Update release manifest files
• Create new Docker files to test the new OS
• Update the supported OS content on GitHub and marketing sites.

Document updates:

• Add OS prerequisites to the dependencies document
• Update distro install page

[richlander]

Alpine Linux 3.12 is out!

https://twitter.com/alpinelinux/status/1266471727678337024

[Thraka]

Please add an item to update docs

[rbhanda]

Update: We are on track to support Alpine 3.12 in our upcoming June releases, including publishing new container images.

[rbhanda]

Alpine 3.12 container images are available and the Distro is now supported with the latest Core release