devenv: Improve nested container support

[cgwalters]

The default containers.conf includes net.ipv4.ping_group_range=0 0 which fails in nested containers because /proc/sys is read-only. This causes 'podman build' to fail with:

open /proc/sys/net/ipv4/ping_group_range: Read-only file system

This matches the approach used by the official quay.io/podman/stable image (containers/image_build) which sets default_sysctls = [] in its podman-containers.conf.

Assisted-by: OpenCode (claude-opus-4-5@20251101)

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the development environment's support for running Podman within another container (podman-in-podman). It specifically addresses and resolves issues related to podman build failures caused by read-only /proc/sys in nested environments, ensuring a more robust and compatible nested container experience by dynamically configuring Podman settings.

Highlights

• Nested Container Support: Improved the development environment's ability to run Podman within another container (podman-in-podman).
• Fix for podman build failures: Resolved issues where podman build failed in nested containers due to a read-only /proc/sys when net.ipv4.ping_group_range was set.
• Dynamic containers.conf Configuration: The userns-setup script now dynamically generates /etc/containers/containers.conf to explicitly set default_sysctls = [], preventing Read-only file system errors.
• Alignment with Upstream: The configuration approach for nested containers now aligns with the official quay.io/podman/stable image.
• Enhanced Documentation: Added a comprehensive 'Nested container support' section to README.md, detailing configuration differences and handling of constrained vs. full UID namespaces.

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[gemini-code-assist]

Code Review

This pull request improves nested container support by disabling default sysctls that cause issues in read-only /proc/sys environments, a problem that can prevent podman build from running. The changes are logical and well-documented in the updated README. My review includes one suggestion to refactor the configure_containers_conf function in devenv/userns-setup to reduce code duplication and improve maintainability.