spec: fetch manifests by tag or digest

Manifests are now fetched by a field called "reference", which may be a tag or
a digest. When using digests to reference a manifest, the data is immutable.
The routes and specification have been updated to allow this.

There are a few caveats to this approach:

1. It may be problematic to rely on data format to differentiate between a tag
   and a digest. Currently, they are disjoint but there may modifications on
   either side that break this guarantee.
2. The caching characteristics of returned content are very different for
   digest versus tag-based references. Digest urls can be cached forever while tag
   urls cannot.

Both of these are minimal caveats that we can live with in the future.

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

docs/api/v2/routes_test.go

@@ -39,16 +39,24 @@ func TestRouter(t *testing.T) {
 			RouteName:  RouteNameManifest,
 			RequestURI: "/v2/foo/manifests/bar",
 			Vars: map[string]string{
-				"name": "foo",
-				"tag":  "bar",
+				"name":      "foo",
+				"reference": "bar",
 			},
 		},
 		{
 			RouteName:  RouteNameManifest,
 			RequestURI: "/v2/foo/bar/manifests/tag",
 			Vars: map[string]string{
-				"name": "foo/bar",
-				"tag":  "tag",
+				"name":      "foo/bar",
+				"reference": "tag",
+			},
+		},
+		{
+			RouteName:  RouteNameManifest,
+			RequestURI: "/v2/foo/bar/manifests/sha256:abcdef01234567890",
+			Vars: map[string]string{
+				"name":      "foo/bar",
+				"reference": "sha256:abcdef01234567890",
 			},
 		},
 		{
@@ -112,8 +120,8 @@ func TestRouter(t *testing.T) {
 			RouteName:  RouteNameManifest,
 			RequestURI: "/v2/foo/bar/manifests/manifests/tags",
 			Vars: map[string]string{
-				"name": "foo/bar/manifests",
-				"tag":  "tags",
+				"name":      "foo/bar/manifests",
+				"reference": "tags",
 			},
 		},
 		{