image manifest: support use as artifact

Implementations lack any information to determine whether the `config` property
of an Image Manifest describes the `artifactType` of the manifest, or the media
type of the referenced content. This is necessitated by the removal of artifact
manifest (#999) and addition of scratch descriptors (#1023) to support
artifacts.

If #999 is withdrawn, this pull request should be withdrawn as well and #1028
considered instead.

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

descriptor.md

@@ -18,7 +18,7 @@ The following fields contain the primary properties that constitute a Descriptor
 
 - **`mediaType`** *string*
 
-  This REQUIRED property MUST either contain the media type of the referenced content, or contain one of the legacy artifact types predating OCI 1.1 described on the config property of [image manifest](manifest.md).
+  This REQUIRED property MUST contain the media type of the referenced content unless this descriptor is the `config` property of an [image manifest](manifest.md).
 
   Values MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2].
 

manifest.md

@@ -33,28 +33,23 @@ Unlike the [image index](image-index.md), which contains information about a set
 
     - **`mediaType`** *string*
 
-        This [descriptor property](descriptor.md#properties) has additional restrictions for `config`.
-        Implementations MUST support at least the following media types:
-
-        - [`application/vnd.oci.image.config.v1+json`](config.md)
-
-        Manifests concerned with portability SHOULD use one of the above media types.
+        This [descriptor property](descriptor.md#properties) has different restrictions for `config`.
+        This value is the `artifactType` property of descriptors referencing this manifest.
 
-        **Legacy artifact types**
+        Implementations MUST NOT attempt to parse the referenced content if this media type is unknown and instead consider the referenced content as arbitrary binary data (e.g.: as `application/octet-stream`).
 
-        Artifacts using [image manifest](manifest.md) MAY set the `mediaType` of their `config` property to one of the following:
+        If this manifest is intended for use by [OCI Runtime Specification](https://github.com/opencontainers/runtime-spec) implementations, the following additional restrictions apply to this property:
 
-        - `application/vnd.cncf.helm.config.v1+json`, describing a Helm Chart artifact
+        Manifests concerned with portability SHOULD use one of the following media types:
 
-        - `application/vnd.sylabs.sif.config.v1+json`, describing a Singularity
-
-        This list may be amended as needed to describe use of image manifest prior to OCI 1.1.
-
-        Implementations SHOULD NOT attempt to parse the referenced content or the layers of unexpected media types, and SHOULD treat unknown data as opaque data (e.g.: `application/octet-stream`).
+        - [`application/vnd.oci.image.config.v1+json`](config.md)
 
 - **`layers`** *array of objects*
 
     Each item in the array MUST be a [descriptor](descriptor.md).
+
+    If this manifest is intended for use by [OCI Runtime Specification](https://github.com/opencontainers/runtime-spec) implementations, the following additional restrictions apply to this property:
+
     The array MUST have the base layer at index 0.
     Subsequent layers MUST then follow in stack order (i.e. from `layers[0]` to `layers[len(layers)-1]`).
     The final filesystem layout MUST match the result of [applying](layer.md#applying-changesets) the layers to an empty directory.
@@ -73,7 +68,6 @@ Unlike the [image index](image-index.md), which contains information about a set
         - [`application/vnd.oci.image.layer.nondistributable.v1.tar+gzip`](layer.md#gzip-media-types)
 
         Manifests concerned with portability SHOULD use one of the above media types.
-        An encountered `mediaType` that is unknown to the implementation MUST NOT generate an error.
 
         Entries in this field will frequently use the `+gzip` types.