Update Alpine ARM docker image

[janvorli]

The Alpine ARM docker image is used for cross compilation. There was a
breaking change in Alpine 3.13 - it uses a new MUSL that has 64 bit
time_t even on 32 bit platforms. Since the official support for Alpine
ARM was not announced yet, it was decided that we will support it only
on Alpine >= 3.13 instead of having to build and distribute two different
versions of runtime.

This change updates the docker image used to build Alpine ARM runtime to
a new version that uses Alpine 3.13 rootfs. It is also using Ubuntu
16.04 instead of 18.04 as a host to unify it with the one we use to
build for Alpine ARM64.

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Issue Details

---

The Alpine ARM docker image is used for cross compilation. There was a
breaking change in Alpine 3.13 - it uses a new MUSL that has 64 bit
time_t even on 32 bit platforms. Since the official support for Alpine
ARM was not announced yet, it was decided that we will support it only
on Alpine >= 3.13 instead of having to build and distribute two different
versions of runtime.

This change updates the docker image used to build Alpine ARM runtime to
a new version that uses Alpine 3.13 rootfs. It is also using Ubuntu
16.04 instead of 18.04 as a host to unify it with the one we use to
build for Alpine ARM64.

Author:	janvorli
Assignees:	janvorli
Labels:	area-Infrastructure-coreclr
Milestone:	6.0.0

[janvorli]

Ah, we'll also need to switch the testing OS for Alpine ARM to Alpine 3.13.

[janvorli]

I have a pending PR dotnet/dotnet-buildtools-prereqs-docker#438 for creating Helix Alpine 3.13 ARM image. After that gets in and the related docker image is produced, I'll update this PR.

[janvorli]

@hoyosjs can you please take a look?

[hoyosjs]

Other than that question, looks good.

[hoyosjs]

Any reason we downgraded to 16.04? This will go out of support this month.

[janvorli]

Yes, the reason was that cross tools (e.g. crossgen targetting arm, but running on x64) would crash on 16.04 and we also use 16.04 on arm64 musl. I guess that when 16.04 goes out of support, we will need to change all 16.04 based images to 18.04 ones.
Ideally, we would run cross build on CentOS 7, but unfortunately that doesn't work due to some differences in default search paths for libs / headers that are compiled into the llvm tools. I have tried to make it work on CentOS 7 in the past, but I didn't have time to finish it. I'll try to dig out that experimental changes once I get some priority things off my table and see if I can make it work.

[janvorli]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/753362431

[github-actions]

@janvorli backporting to release/5.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Update Alpine ARM docker image
Using index info to reconstruct a base tree...
M	eng/pipelines/common/platform-matrix.yml
Falling back to patching base and 3-way merge...
Auto-merging eng/pipelines/common/platform-matrix.yml
Applying: Update helix queue for Alpine ARM to use Alpine 3.13
Using index info to reconstruct a base tree...
M	eng/pipelines/coreclr/templates/helix-queues-setup.yml
Falling back to patching base and 3-way merge...
Auto-merging eng/pipelines/coreclr/templates/helix-queues-setup.yml
CONFLICT (content): Merge conflict in eng/pipelines/coreclr/templates/helix-queues-setup.yml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0002 Update helix queue for Alpine ARM to use Alpine 3.13
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!