ci: Alpine Linux Docker image (musl)

[jpnurmi]

This PR adds a workflow for building an Alpine Linux Docker image tailored for the CI workflows in sentry-dotnet, which is a prerequisite for testing linux-musl-x64 in:

• fix: support musl on Linux  #4188

Background

The official Alpine Linux Docker image is minimal (< 5MB!) and requires quite a bit of boilerplate to make it usable even for basic CI workflows:

• git must be installed before actions/checkout
• tar must be installed before actions/cache
• bash must be installed before any shell: bash steps
• ...

Furthermore, the ready-made runner images come with a wide range of pre-installed development packages and tools missing in the vanilla Alpine Linux image. Last but not least, various setup-xxx actions rely on relaxed system permissions (chmod 777 -R /opt /usr/share), and the integration test composite action requires a runner-like structure.

Notes

• The workflow requires packages: write permission to push to ghcr.io.
• This action only needs to be triggered once, but has a trigger on .github/alpine should the Dockerfile ever need adjustments.

#skip-changelog

[jpnurmi]

P.S. The Docker image is temporarily published from my fork for testing purposes in #4188:

https://github.com/jpnurmi/sentry-dotnet/pkgs/container/sentry-dotnet-alpine

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.98%. Comparing base (495e742) to head (9a9ae9b).
Report is 595 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4190      +/-   ##
==========================================
- Coverage   75.73%   72.98%   -2.75%     
==========================================
  Files         357      452      +95     
  Lines       13466    16439    +2973     
  Branches     2671     3283     +612     
==========================================
+ Hits        10198    11998    +1800     
- Misses       2593     3605    +1012     
- Partials      675      836     +161     

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[bruno-garcia]

Running: https://github.com/getsentry/sentry-dotnet/actions/runs/15087760535/job/42412495651

[bruno-garcia]

worked! yeee
https://github.com/getsentry/sentry-dotnet/pkgs/container/sentry-dotnet-alpine

[jpnurmi]

Awesome :) Thanks!