layers: describe policy enforcement for foreign layer media type

Signed-off-by: Stephen J Day <stephen.day@docker.com>

layer.md

@@ -156,3 +156,13 @@ Note that this opaque file will apply to _all_ children, including sub-directori
 Implementations SHOULD generate layers using _explicit whiteout_ files, but MUST accept both.
 
 Any given image is likely to be composed of several of these Image Filesystem Changeset tar archives.
+
+# Foreign Layers
+
+Certain layers, due to legal requirements, may not be regularly distributable.
+Typically, one can download such layers but they should never be uploaded.
+
+Layers that have these restrictions SHOULD be tagged with an alternative mediatype of `application/vnd.oci.image.serialization.rootfs.foreign.tar.gzip`.
+[Descriptors][descriptor.md] referencing these layers MAY include `urls` for downloading these layers.
+Implementations MUST NOT distribute layers tagged with this media type.
+

media-types.md

@@ -7,6 +7,7 @@ The following `mediaType` MIME types are used by the formats described here, and
 - `application/vnd.oci.image.manifest.v1+json`: [Image manifest format](manifest.md#image-manifest)
 - `application/vnd.oci.image.serialization.config.v1+json`: [Container config JSON](serialization.md)
 - `application/vnd.oci.image.serialization.rootfs.tar.gzip`: ["Layer", as a gzipped tar archive](layer.md)
+- `application/vnd.oci.image.serialization.rootfs.foreign.tar.gzip`: ["Layer", as a gzipped tar that should never be pushed](serialization.md#foreign-layers)
 
 ## Compatibility Matrix