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

Closes: #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].
@@ -331,3 +336,4 @@ Implementations SHOULD NOT upload layers tagged with this media type; however, s
 [gnu-tar-standard]: http://www.gnu.org/software/tar/manual/html_node/Standard.html
 [rfc1952_2]: https://tools.ietf.org/html/rfc1952
 [tar-archive]: https://en.wikipedia.org/wiki/Tar_(computing)
+[rfc8478]: https://tools.ietf.org/html/rfc8478

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
 
@@ -66,3 +68,4 @@ The following figure shows how the above media types reference each other:
 The image-index being a "fat manifest" references a list of image manifests per target platform. An image manifest references exactly one target configuration and possibly many layers.
 
 [rfc1952]: https://tools.ietf.org/html/rfc1952
+[rfc8478]: https://tools.ietf.org/html/rfc8478