manifest: Explicitly make 'layers' optional

Most folks will distribute images containing layers, but the specified
behavior applies cleanly to the layer-less case too.  The unpacked
rootfs will just be an empty directory with unspecified attributes.
Folks might want to use that sort of thing for namespace pinning [1],
distributing configs [2], setting up containers that mount the meat
from the host [2], etc.

[1]: opencontainers/runtime-spec#395 (comment)
[2]: #313 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

manifest.md

@@ -139,6 +139,7 @@ Unlike the [Manifest List](#manifest-list), which contains information about a s
 
 - **`layers`** *array*
 
+    This OPTIONAL property references the [layers](layer.md) defining the container's root filesystem.
     Each item in the array MUST be a [descriptor](descriptor.md).
     The root filesystem MUST match an empty directory onto which the layers have been [applied](layer.md#applying) in the listed order.
     The [ownership, mode, and other attributes](layer.md#file-attributes) of the initial empty directory are unspecified.

schema/image-manifest-schema.json

@@ -30,7 +30,6 @@
   "required": [
     "schemaVersion",
     "mediaType",
-    "config",
-    "layers"
+    "config"
   ]
 }

specs-go/v1/manifest.go

@@ -25,7 +25,7 @@ type Manifest struct {
 	Config specs.Descriptor `json:"config"`
 
 	// Layers is an indexed list of layers referenced by the manifest.
-	Layers []specs.Descriptor `json:"layers"`
+	Layers []specs.Descriptor `json:"layers,omitempty"`
 
 	// Annotations contains arbitrary metadata for the manifest list.
 	Annotations map[string]string `json:"annotations"`