Support zstd compression in image commit

Without this change, specifying `Compression: imagebuildah.Zstd` in
`imagebuildah`'s `BuildOptions fails, so it is not possible to push
cache to a registry with zstd compression.

Note this is only applicable to OCI manifests.

Signed-off-by: Aaron Lehmann <[email protected]>

Author: Aaron Lehmann

image.go

@@ -621,9 +621,8 @@ func (mb *ociManifestBuilder) computeLayerMIMEType(what string, layerCompression
 			// how to decompress them, we can't try to compress layers with xz.
 			return errors.New("media type for xz-compressed layers is not defined")
 		case archive.Zstd:
-			// Until the image specs define a media type for zstd-compressed layers, even if we know
-			// how to decompress them, we can't try to compress layers with zstd.
-			return errors.New("media type for zstd-compressed layers is not defined")
+			omediaType = v1.MediaTypeImageLayerZstd
+			logrus.Debugf("compressing %s with zstd", what)
 		default:
 			logrus.Debugf("compressing %s with unknown compressor(?)", what)
 		}