engine: add platform field to EngineConfig

[TrevorBurnham]

Issue: containers/podman#25641

This PR adds a new platform option to the [engine] section of containers.conf:

[engine]
platform = "linux/amd64"

This allows users to set a default platform (os/arch or os/arch/variant) for image operations such as pull, build, run, and create.

Why

Users currently have no way to globally default to a different platform. This forces workarounds for tools that don't expose --platform flags. Docker supports this via DOCKER_DEFAULT_PLATFORM, but Podman has no equivalent config or env var.

This PR provides the containers/common foundation. A follow-up PR in containers/podman will wire this into pull/build/run/create and add DOCKER_DEFAULT_PLATFORM env var support for Docker compatibility.

Changes

• pkg/config/config.go: Added Platform field to EngineConfig with toml:"platform" tag
• pkg/config/config.go: Added PlatformComponents() method to parse the field into os, arch, variant
• pkg/config/config.go: Added validatePlatform() and wired it into EngineConfig.Validate()
• docs/containers.conf.5.md: Documented the new platform option in the [engine] table
• pkg/config/config_test.go: Added tests for valid/invalid platforms, empty default, and component parsing
• pkg/config/testdata/containers_override.conf: Added platform to the override test fixture

Validation

The platform string is validated on config load:

• Must be os/arch or os/arch/variant (e.g., linux/amd64, linux/arm64/v8)
• Empty string is valid (means "use host platform")
• Rejects single-component values (linux), trailing slashes (linux/amd64/), and missing components (/amd64)

[github-actions]

This repository has been migrated to https://github.com/containers/container-libs. Please open your PR there.

[TrevorBurnham]

Whoops! Opened as containers/container-libs#669.