[ECR] [feature request]: Inspector support for scan of SIF images

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**Tell us about your request**
Basic and enhanced scanning support for SIF container images.

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
ECR should support scanning of SIF format container images stored in a repository.
Presently, ECR gives message `Scan status: UNSUPPORTED_IMAGE`.

The impact of not having this feature added is that environments with singularity containers will not be able to scan their containers stored in ECR for vulnerabilities.

**Are you currently working around this issue?**
No. It is not currently possible for singularity (apptainer) release to push to ECR using `docker://` transport. It is possible for singularity (apptainer) to pull a container *from* registries using docker protocol, such as public registry.

It is currently possible to push images to ECR in SIF format using `oras://` transport protocol. The singularity command line can then pull these containers stored in ECR. That much is working correctly in ECR with OCI artifacts that match singularity images; when the same runtime is used to both push and pull through ORAS/OCI.

It is currently possible to push images to ECR in docker format and enhanced scanning will work. It is possible to first convert a singularity image to a docker image, and then push it with any container tools which support docker images such as docker or podman container run-time cli.

The reason this feature request should be voted for is because apptainer (singularity) containers are now becoming more common on Linux HPC clusters and having to convert between image formats in order to push to ECR for enhanced scanning to work, is problematic for users.

**Additional context**
The singularity image format (SIF) uses ORAS to push containers to ECR and stores the file system layers as OCI artifacts of type: ```application/vnd.sylabs.sif.layer.v1.sif```.

- https://github.com/opencontainers/artifacts/blob/main/artifact-authors.md#defining-layermediatypes
- [Apptainer: The Singularity Image Format (SIF) (GitHub)](https://github.com/apptainer/sif)

SIF layers typically use [squashfs](https://www.kernel.org/doc/html/next/filesystems/squashfs.html#:~:text=Squashfs%20is%20a%20compressed%20read,packed%20to%20minimise%20data%20overhead.) compressed file system. However, SIF layers may also include overlays and use extfs in older `.simg` container images. Ideally, those additional layers could be included during scan.

- https://github.com/quay/clair/pull/475:
- [clair-singularity](https://github.com/dctrud/clair-singularity) allows clair server to scan singularity format, by first exporting layers as tar.gz. Have not tested this integration.
- https://github.com/singularityhub/stools:
- docker-compose.yml starts clair-scanner in order to scan SIF images using sclair command.

**Attachments**
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The issue names no repository files, tests, or implementation entry points. Start with the reported ECR `UNSUPPORTED_IMAGE` status and the SIF/OCI artifact media type details; done would mean ECR basic and enhanced scanning accepts SIF images, including relevant layers.

Written by the indexing model from the issue text.