add platform flag to store save

[amartin120]

Please check below, if the PR fulfills these requirements:

• Commit(s) and code follow the repositories guidelines.
• Test(s) have been added or updated to support these change(s).
• Doc(s) have been added or updated to support these change(s).

Associated Links:

• bump to cosign v2.2.3-carbide.3 for new annotation #322

Types of Changes:

• bugfix

Proposed Changes:

• An image index a.k.a. multi-arch image isn't really docker compatible so this PR adjusts hauler store save to have a --platform flag to give the user a chance to only export the platform of their choice. Any platform skipped as a result of this flag will be logged as a warning.

• If the --platform flag isn't provided, a warning gets logged during save if your store contains an image index... just as a heads up.

• Docker also isn't compatible with the unknown/unknown platform that some image index might contain. we're going to skip those or otherwise docker load will fail to import.

Verification/Testing of Changes:

• example of omitting the platform flag and hauler just throwing a warning when it processes an image index.

> hauler store save                                                                                                                                                                                                                                                           
2024-09-23 15:18:20 WRN index [index.docker.io/bitnami/cert-manager:1.15.3]: contains multiple platforms and could cause issues when importing into docker/containerd

• example of using the --platform flag to skip anything other than what's specified.

> hauler store save --platform linux/arm64                                                                                                                                                                                                                                     
2024-09-23 15:19:05 WRN index [index.docker.io/bitnami/cert-manager:1.15.3]: digest=sha256:5c827d71196f865e168e3fed85ed1bd54863fc8f01760a833253474aed4a1441, platform=linux/amd64: does not match the supplied platform, skipping
2024-09-23 15:19:05 INF saved store [store] -> [/Users/amartin/projects/hauler/haul.tar.zst]

• example of an image index containing unknown/unknown

2024-09-23 15:18:20 WRN index [index.docker.io/library/busybox:latest]: digest=sha256:c230832bd3b0be59a6c47ed64294f9ce71e91b327957920b6929a0caa8353140, platform=unknown/unknown: skipping 'unknown/unknown' platform

Additional Context:

[amartin120]

@dweomer I'm second guessing the flag being named platform. It might be mistaken for the entire save operation when ultimately it's just the 'manifest.json'.

How about --manifest-platform or --docker-platform? Thoughts?

[dweomer]

I think --platform is right and consistent with other subcommand usages.