fix: respect OCIRuntime field in SpecGenerator

[GiulioSavini]

Description

When creating containers via the API with OCIRuntime set in the SpecGenerator, the field was silently ignored. The container always used the default runtime from containers.conf instead of the one specified in the API request.

The issue is in pkg/specgen/generate/container_create.go at line 168: the code only considered the image-platform-based runtime selection (ImagePlatformToRuntime) but never checked s.OCIRuntime.

Changes

The fix gives the user's explicit OCIRuntime choice priority over the automatic image-platform-based selection, and falls back to the existing behavior when not set:

if s.OCIRuntime != "" {
    options = append(options, libpod.WithCtrOCIRuntime(s.OCIRuntime))
} else if imageData != nil {
    // existing image-platform-based logic
}

This is consistent with how the CLI --runtime flag works, which correctly passes the runtime through.

Test plan

• Create container via API with OCIRuntime set to a non-default runtime
• Verify the container uses the specified runtime instead of the default
• Create container via API without OCIRuntime set — verify existing behavior unchanged
• Verify --runtime CLI flag still works as before

Fixes #28429

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[Luap99]

There is already someone working on this in #28454, please do not work on issues that already have open PRs and rather review them if you are interest in that