Stronger validation for uuid field in urls

This change adds strong validation for the uuid variable for v2 routes. This is
a minor specification change but is okay since the uuid field is controlled by
the server. The character set is restricted to avoid path traversal, allowing
for alphanumeric values and urlsafe base64 encoding.

This change has no effect on client implementations.

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

docs/api/v2/routes_test.go

@@ -98,6 +98,7 @@ func TestRouter(t *testing.T) {
 			},
 		},
 		{
+			// support uuid proper
 			RouteName:  RouteNameBlobUploadChunk,
 			RequestURI: "/v2/foo/bar/blobs/uploads/D95306FA-FAD3-4E36-8D41-CF1C93EF8286",
 			Vars: map[string]string{
@@ -113,6 +114,21 @@ func TestRouter(t *testing.T) {
 				"uuid": "RDk1MzA2RkEtRkFEMy00RTM2LThENDEtQ0YxQzkzRUY4Mjg2IA==",
 			},
 		},
+		{
+			// supports urlsafe base64
+			RouteName:  RouteNameBlobUploadChunk,
+			RequestURI: "/v2/foo/bar/blobs/uploads/RDk1MzA2RkEtRkFEMy00RTM2LThENDEtQ0YxQzkzRUY4Mjg2IA_-==",
+			Vars: map[string]string{
+				"name": "foo/bar",
+				"uuid": "RDk1MzA2RkEtRkFEMy00RTM2LThENDEtQ0YxQzkzRUY4Mjg2IA_-==",
+			},
+		},
+		{
+			// does not match
+			RouteName:  RouteNameBlobUploadChunk,
+			RequestURI: "/v2/foo/bar/blobs/uploads/totalandcompletejunk++$$-==",
+			StatusCode: http.StatusNotFound,
+		},
 		{
 			// Check ambiguity: ensure we can distinguish between tags for
 			// "foo/bar/image/image" and image for "foo/bar/image" with tag