cc @ViktorHofer, @janvorli based on https://github.com/dotnet/core/blob/main/release-notes/6.0/supported-os.md#linux, we are currently only supporting Alpine 3.13 onwards. This PR sets default version to 3.13. I will send a subsequent PR to dotnet-buildtools-prereqs-docker so we get updated tags for both crossbuild arm (which is currently using 3.13) and crossbuild arm64 (which is currently using 3.9). We have already updated WithNode and helix to 3.13 and added 3.14 in docker matrix.

@ViktorHofer, this and dotnet/runtime#57324 are related (but not dependent) and ready for review/merge. :)

dotnet/runtime#57324 is:

• WithNode: used for building x64 binaries.
• helix: used for testing x64, arm64, arm.

This PR + the future dotnet-prereq PRs with this change and runtime PRs consuming the updated Ubuntu image:

• crossbuild: used for cross-building arm64 and arm binaries.

so it turned out, that "alpine update" story has a long tail than I imagined. 😅

Thanks for the great work again @am11 👍