855 attest registry source only (#856)

Add source.NewFromRegistry function so that the syft attest command can always explicitly ask for an OCIRegistry provider rather than rely on local daemon detection for image sources.

Attestation can not be used where local images loaded in a daemon are the source. Digest values for the layer identification step in attestation can sometimes vary across workstations.

This fix makes it so that attest is generating an SBOM for, and attesting to, a source that exists in an OCI registry. It should never load a source from a local user docker/podman daemon.

Signed-off-by: Christopher Phillips <christopher.phillips@anchore.com>
Co-authored-by: Alex Goodman <alex.goodman@anchore.com>

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/anchore/go-testutils v0.0.0-20200925183923-d5f45b0d3c04
 	github.com/anchore/go-version v1.2.2-0.20200701162849-18adb9c92b9b
 	github.com/anchore/packageurl-go v0.0.0-20210922164639-b3fa992ebd29
-	github.com/anchore/stereoscope v0.0.0-20220217141419-c6f02aed9ed2
+	github.com/anchore/stereoscope v0.0.0-20220301220648-8aa8a4a0bf50
 	github.com/antihax/optional v1.0.0
 	github.com/bmatcuk/doublestar/v4 v4.0.2
 	github.com/docker/docker v20.10.12+incompatible
@@ -285,7 +285,7 @@ require (
 	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/golang/snappy v0.0.4 // indirect
-	github.com/google/go-containerregistry v0.8.1-0.20220209165246-a44adc326839
+	github.com/google/go-containerregistry v0.8.1-0.20220209165246-a44adc326839 // indirect
 	github.com/hashicorp/errwrap v1.1.0 // indirect
 	github.com/hashicorp/hcl v1.0.0 // indirect
 	github.com/iancoleman/orderedmap v0.0.0-20190318233801-ac98e3ecb4b0 // indirect