Add support for Ubuntu 21.04 (Hirsute Hippo)

[mthalman]

New Linux Distro Version

Distro: Ubuntu 21.04 (Hirsute Hippo)

ImageBuilder Tasks

• Ensure that the ImageBuilder supports the new distro version in the code to generate the correct README display name from the version specified in the manifest

Nightly Branch Tasks

1. • Copy the Dockerfiles of the most recent version of that distro (or author new ones for an entirely new distro) and place them in a version-specific folder under their respective variants (runtime-deps, runtime, aspnet, sdk). Note: not all variants have a corresponding runtime-deps image.
2. • Modify the Dockerfiles as appropriate for any specific changes related to the new distro version
3. • Update manifest.json to reference the new set of Dockerfiles with the appropriate tags
     • Move any distro-specific floating tags to the newer version (e.g. 3.1-alpine)
4. • Update the test data to include the new distro version
5. • Update the tags metadata templates to include the new distro version
6. • Run the command to update the READMEs: .\eng\readme-templates\Get-GeneratedReadmes.ps1
7. • Run the command to update the image size baseline file: .\tests\performance\Validate-ImageSize.ps1 -UpdateBaselines
8. • Inspect generated changes for correctness
9. • Consider whether sample Dockerfiles should be authored if this is a new distro and them to the samples
10. • Run the command to build and test your changes: .build-and-test.ps1 -OS <os>
11. • Commit generated changes
12. • Create PR
13. • Get PR signoff
14. • Merge PR to nightly branch
15. • Wait for automatically queued CI build to finish on dotnet-docker-nightly pipeline (internal MSFT link)
16. • Confirm images have been ingested by MCR
17. • Confirm READMEs have been updated in Docker Hub

Master Branch Tasks

1. • After the product teams have signed off on the new distro, merge these changes to the master branch as part of the release process for the next .NET release
2. • Create an announcement (example: Alpine 3.10) unless the new distro is added only for pre-release versions in which the announcement would be incorporated in the pre-release notes.

[mthalman]

Related to dotnet/core#5850

[MichaelSimons]

Ubuntu 21.04 (Hirsute Hippo) is not an LTS. The .NET Docker OS policy states only Ubuntu LTS will be supported.

[mthalman]

Whoops, I thought it was LTS but I see that LTS is only released every two years. Closing.