Make /alertmanager group-writable in Dockerfile

[VaibhavOza1997]

This change makes the /alertmanager directory group-writable in the Dockerfile.

It follows the precedent set in prometheus/prometheus#16073 to support running the Alertmanager container as an arbitrary user. This aligns with best practices for container security and facilitates use cases where the container is run without root privileges.

Fixes: #4436

[VaibhavOza1997]

Hi maintainers 👋,

This is my first contribution to the project. This PR makes the /alertmanager directory group-writable in the Dockerfile, following the precedent from prometheus/prometheus#16073. This change allows running Alertmanager containers as arbitrary non-root users.

All feedback is very welcome. Thank you for your time and for maintaining this awesome project!

Best regards,
Vaibhav Oza

[TheMeier]

hmm usually you use mounted volumes for that also best practice is to run production workload with read-only for the container fs. So doesn't seem to make much sense to me

[VaibhavOza1997]

Thank you for the quick feedback!

That makes sense. I see your point about using mounted volumes and keeping the container filesystem read-only in production. The motivation for this change came from issue #4436, where making /alertmanager group-writable was suggested to support running the container as an arbitrary user without needing to adjust volume permissions externally. I’m happy to close this PR if you think this approach doesn’t align with the project’s best practices. Just wanted to try to help based on the open issue. 😊