stages/oci-archive: define ostree annotations

Define a set of pre-defined ostree related annotations that can
and should be used to indicate that a container image contains
an OSTree commit. This can be used by other tools to inspect and
extract the commit more easily.

stages/org.osbuild.oci-archive

@@ -20,6 +20,14 @@ Manifest annotations can be included via the `annotation` options. Any
 `key`, `value` pair is allowed, with the exception of the pre-defined
 `org.osbuild` and `org.opencontainer` namespaces.
 
+Specific annotations can be used to indicate that a container contains
+an OSTree commit via the following:
+  - `org.osbuild.layer`: The layer containing the OSTree repository
+  - `org.osbuild.repo`: Path inside the container to the repository
+  - `org.osbuild.ref`: OSTree reference of the commit inside the repo
+The `org.osbuild.layer` value can either bey a index (starting at 0),
+or a digest in the form of <algorithm>:<checksum>.
+
 The final resulting tarball, aka a "orci-archive", can be imported via
 podman[3] with `podman pull oci-archive:<archive>`.
 
@@ -103,6 +111,20 @@ SCHEMA_2 = r"""
     },
     "annotations": {
       "type": "object",
+      "properties": {
+        "org.osbuild.ostree.layer": {
+          "description": "The layer that contains the OSTree repository",
+          "type": "string"
+        },
+        "org.osbuild.ostree.repo": {
+          "description": "Path to the OSTree repository inside the layer",
+          "type": "string"
+        },
+        "org.osbuild.ostree.ref": {
+          "description": "Reference of the OSTree commit in the repository",
+          "type": "string"
+        }
+      },
       "additionalProperties": false,
       "patternProperties": {
         "^(?!org.osbuild|org.opencontainer).+": {