Ensure golden images do not return digests

[wagoodman]

Currently the GetFixtureImage will return the build image intact, however, the digests should be stubbed out as they will always change (https://app.circleci.com/pipelines/github/anchore/imgbom/29/workflows/e36d826e-0885-4408-bdbc-a3413c2db023/jobs/70/steps)

[alfredodeza]

The layer information is used in many outputs, so the digest (here on imgbom text ouput) changes:

         Layer:		 1
         Digest:	 sha256:9b437d7d61c48116592c57e0a646fed378641d5b09662a8172e168b7938cf3147af478720cdc9bd4
         Size:		 1876
         MediaType:	 application/vnd.docker.image.rootfs.diff.tar.gzip

The Image.Metadata.Digest contains the value that needs to be stubbed out:

	for _, l := range img.Layers {
		l.Metadata.Digest = "sha256:ad8ecdc058976c07e7e347cb89fa9ad86a294b5ceaae6d09713fb035f84115abf3c4a2388a4af3aa60f13b94f4c6846930bdf53"
	}

[alfredodeza]

@wagoodman should these values always be stubbed out?

[wagoodman]

fair question, I'm not certain. I know for images that are not committed to the repo, this will be an unstable field, thus, always useless. However, I think that GetGoldenFixtureImage uses GetFixtureImage, where the image is committed to the git repo, and the saved digest is relevant and stable.

When implementing this we'll have to keep this in mind, otherwise I think it is safe to assume that these fields should always be stubbed. Maybe provide another helper where this behavior is more apparent in the name?

[alfredodeza]

Another helper does sound like the best path forward here, given that we can't have keyword arguments with defaults in Go.