Allow for override of authorization scope

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

README.md

@@ -84,6 +84,15 @@ Here is a visual of this auth flow copied from the [Docker docs](https://docs.do
 
 ![](./v2-registry-auth.png)
 
+#### Custom Auth Scope
+
+ It may be necessary to override the `scope` obtained from the `Www-Authenticate` header in the registry's response. This can be done on the client level:
+
+ ```
+client, err := reggie.NewClient("http://localhost:5000",
+    reggie.WithAuthScope("repository:mystuff/myrepo:pull,push"))
+ ```
+
 ## Other Features
 
 ### Method Chaining

auth.go

@@ -44,9 +44,12 @@ func (client *Client) retryRequestWithAuth(originalRequest *Request, originalRes
 			SetHeader("Accept", "application/json").
 			SetHeader("User-Agent", client.Config.UserAgent).
 			SetBasicAuth(client.Config.Username, client.Config.Password)
-		if h.Scope != "" {
+		if s := client.Config.AuthScope; s != "" {
+			req.SetQueryParam("scope", s)
+		} else if h.Scope != "" {
 			req.SetQueryParam("scope", h.Scope)
 		}
+
 		authResp, err := req.Execute(GET, h.Realm)
 		if err != nil {
 			return nil, err

client.go

@@ -24,6 +24,7 @@ type (
 
 	clientConfig struct {
 		Address     string
+		AuthScope   string
 		Username    string
 		Password    string
 		Debug       bool
@@ -65,6 +66,13 @@ func WithUsernamePassword(username string, password string) clientOption {
 	}
 }
 
+// WithAuthScope overrides the scope provided by the authorization server.
+func WithAuthScope(authScope string) clientOption {
+	return func(c *clientConfig) {
+		c.AuthScope = authScope
+	}
+}
+
 // WithDefaultName sets the default registry namespace configuration setting.
 func WithDefaultName(namespace string) clientOption {
 	return func(c *clientConfig) {

client_test.go

@@ -79,6 +79,17 @@ func TestClient(t *testing.T) {
 		t.Errorf("Setting the debug flag didn't work")
 	}
 
+	// test setting auth scope
+	testScope := `realm="https://silly.com/v2/auth",service="testservice",scope="pull,push"`
+	client3, err := NewClient(registryTestServer.URL, WithAuthScope(testScope))
+	if err != nil {
+		t.Fatalf("Errors creating client: %s", err)
+	}
+
+	if s := client3.Config.AuthScope; s != testScope {
+		t.Errorf("Setting the auth scope didn't work: %s", s)
+	}
+
 	// test default name
 	req := client.NewRequest(GET, "/v2/<name>/tags/list")
 	if !strings.HasSuffix(req.URL, "/v2/testname/tags/list") {
@@ -99,8 +110,6 @@ func TestClient(t *testing.T) {
 		t.Fatalf("Expected response code 200 but was %d", status)
 	}
 
-
-
 	// test default name reset
 	client.SetDefaultName("othername")
 	req = client.NewRequest(GET, "/v2/<name>/tags/list")