Add Alpine 3.21 images to main branch

[lbussell]

Part of [Alpine 3.21] Create new container images (dotnet/core#9579)

Follow-up to: Add Alpine 3.21 images to nightly branch (dotnet/dotnet-docker#6095)

Alpine 3.21 images should be published from the main branch with the next .NET servicing release.

[richlander]

I wonder if we should change our policy to make all new distro versions available as soon as we are able. We have done day of releases for Ubuntu and Azure Linux. It would be good to have a consistent approach.

[MichaelSimons]

I wonder if we should change our policy to make all new distro versions available as soon as we are able. We have done day of releases for Ubuntu and Azure Linux. It would be good to have a consistent approach.

I think that should be the goal. This is currently blocked on product signoff that the newest version is supported.

[lbussell]

I wonder if we should change our policy to make all new distro versions available as soon as we are able. We have done day of releases for Ubuntu and Azure Linux. It would be good to have a consistent approach.

[Alpine 3.21] is currently blocked on product signoff that the newest version is supported.

I think that the only way we could get day-of Alpine support is if we continuously tested all supported .NET versions on alpine:edge images.

[richlander]

The request wasn't "day of" support but "as soon as we are able". I'm going to address this with some other folks.

[lbussell]

These are available as of this week. Announcement: #6146