[am11]

Align Alpine versions in cross-build.

before:

$ ARM32_URN=mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm-alpine-20210719121212-044d5b9
$ ARM64_URN=mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-alpine-20210719121212-b2c2436

$ docker run $ARM32_URN grep VERSION /crossrootfs/arm/etc/os-release
VERSION_ID=3.13.5

$ docker run $ARM64_URN grep VERSION /crossrootfs/arm64/etc/os-release
VERSION_ID=3.9.6

after:

$ ARM32_URN=mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm-alpine-20210923140502-78f7860
$ ARM64_URN=mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-alpine-20210923140502-78f7860

$ docker run $ARM32_URN grep VERSION /crossrootfs/arm/etc/os-release
VERSION_ID=3.13.6

$ docker run $ARM64_URN grep VERSION /crossrootfs/arm64/etc/os-release
VERSION_ID=3.13.6

cc @janvorli

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[janvorli]

LGTM, thank you!

[hoyosjs]

Regex failure is well known.

[am11]

@janvorli, is it good to merge?