media-types: Define layer media types suffix '+zstd'

Closes: opencontainers#787

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

layer.md

@@ -4,13 +4,18 @@ This document describes how to serialize a filesystem and filesystem changes lik
 One or more layers are applied on top of each other to create a complete filesystem.
 This document will use a concrete example to illustrate how to create and consume these filesystem layers.
 
-This section defines the `application/vnd.oci.image.layer.v1.tar`, `application/vnd.oci.image.layer.v1.tar+gzip`, `application/vnd.oci.image.layer.nondistributable.v1.tar`, and `application/vnd.oci.image.layer.nondistributable.v1.tar+gzip` [media types](media-types.md).
+This section defines the `application/vnd.oci.image.layer.v1.tar`, `application/vnd.oci.image.layer.v1.tar+gzip`, `application/vnd.oci.image.layer.v1.tar+zstd`, `application/vnd.oci.image.layer.nondistributable.v1.tar`, `application/vnd.oci.image.layer.nondistributable.v1.tar+gzip`, and `application/vnd.oci.image.layer.nondistributable.v1.tar+zstd` [media types](media-types.md).
 
 ## `+gzip` Media Types
 
 * The media type `application/vnd.oci.image.layer.v1.tar+gzip` represents an `application/vnd.oci.image.layer.v1.tar` payload which has been compressed with [gzip][rfc1952_2].
 * The media type `application/vnd.oci.image.layer.nondistributable.v1.tar+gzip` represents an `application/vnd.oci.image.layer.nondistributable.v1.tar` payload which has been compressed with [gzip][rfc1952_2].
 
+## `+zstd` Media Types
+
+* The media type `application/vnd.oci.image.layer.v1.tar+zstd` represents an `application/vnd.oci.image.layer.v1.tar` payload which has been compressed with [zstd][rfc8478].
+* The media type `application/vnd.oci.image.layer.nondistributable.v1.tar+zstd` represents an `application/vnd.oci.image.layer.nondistributable.v1.tar` payload which has been compressed with [zstd][rfc8478].
+
 ## Distributable Format
 
 * Layer Changesets for the [media type](media-types.md) `application/vnd.oci.image.layer.v1.tar` MUST be packaged in [tar archive][tar-archive].

media-types.md

@@ -9,8 +9,10 @@ The following media types identify the formats described here and their referenc
 - `application/vnd.oci.image.config.v1+json`: [Image config](config.md)
 - `application/vnd.oci.image.layer.v1.tar`: ["Layer", as a tar archive](layer.md)
 - `application/vnd.oci.image.layer.v1.tar+gzip`: ["Layer", as a tar archive](layer.md#gzip-media-types) compressed with [gzip][rfc1952]
+- `application/vnd.oci.image.layer.v1.tar+zstd`: ["Layer", as a tar archive](layer.md#zstd-media-types) compressed with [zstd][rfc8478]
 - `application/vnd.oci.image.layer.nondistributable.v1.tar`: ["Layer", as a tar archive with distribution restrictions](layer.md#non-distributable-layers)
 - `application/vnd.oci.image.layer.nondistributable.v1.tar+gzip`: ["Layer", as a tar archive with distribution restrictions](layer.md#gzip-media-types) compressed with [gzip][rfc1952]
+- `application/vnd.oci.image.layer.nondistributable.v1.tar+zstd`: ["Layer", as a tar archive with distribution restrictions](layer.md#zstd-media-types) compressed with [zstd][rfc8478]
 
 ## Media Type Conflicts
 
@@ -50,6 +52,12 @@ This section shows where the OCI Image Specification is compatible with formats
 
 - [application/vnd.docker.image.rootfs.diff.tar.gzip](https://github.com/docker/docker/blob/master/image/spec/v1.md#creating-an-image-filesystem-changeset)
 
+### application/vnd.oci.image.layer.v1.tar+zstd
+
+**Interchangeable and fully compatible mime-types**
+
+- [application/vnd.docker.image.rootfs.diff.tar.zstd](https://github.com/docker/docker/blob/master/image/spec/v1.md#creating-an-image-filesystem-changeset)
+
 ### application/vnd.oci.image.config.v1+json
 
 **Similar/related schema**