Allow explicit docker login errors to be ignored

* Add support for suppressing errors triggered by explicit calls to
  `docker login` and `docker logout` by setting the
  `IGNORE_DOCKER_LOGIN` environment variable.

README.md

@@ -39,6 +39,8 @@ The `docker-credential-env` binary must be installed to `$PATH`, configured via
   }
   ```
 
+By default, attempts to explicitly `docker {login,logout}` will generate an error. To ignore these errors, set the environment variable `IGNORE_DOCKER_LOGIN=1`.
+
 ## Example Usage
 
 ### Jenkins

env.go

@@ -26,6 +26,7 @@ const (
 	envUsernameSuffix = "USR"
 	envPasswordSuffix = "PSW"
 	envSeparator      = "_"
+	envIgnoreLogin    = "IGNORE_DOCKER_LOGIN"
 )
 
 type NotSupportedError struct{}
@@ -39,12 +40,22 @@ type Env struct{}
 
 // Add implements the set verb
 func (*Env) Add(*docker_credentials.Credentials) error {
-	return fmt.Errorf("add: %w", &NotSupportedError{})
+	switch {
+	case os.Getenv(envIgnoreLogin) != "":
+		return nil
+	default:
+		return fmt.Errorf("add: %w", &NotSupportedError{})
+	}
 }
 
 // Delete implements the erase verb
 func (*Env) Delete(string) error {
-	return fmt.Errorf("delete: %w", &NotSupportedError{})
+	switch {
+	case os.Getenv(envIgnoreLogin) != "":
+		return nil
+	default:
+		return fmt.Errorf("delete: %w", &NotSupportedError{})
+	}
 }
 
 // List implements the list verb

env_test.go

@@ -213,17 +213,33 @@ func TestEnvNotSupportedMethods(t *testing.T) {
 		}
 	})
 
+	t.Run("Add is ignored with IGNORE_DOCKER_LOGIN", func(t *testing.T) {
+		t.Setenv(envIgnoreLogin, "1")
+		actualErr := e.Add(nil)
+		if actualErr != nil {
+			t.Errorf("Add() actual = (%v), expected (%v)", actualErr, nil)
+		}
+	})
+
 	t.Run("Delete is not supported", func(t *testing.T) {
 		actualErr := e.Delete("")
 		if !errors.Is(actualErr, &NotSupportedError{}) {
-			t.Errorf("Add() actual = (%v), expected (%v)", actualErr, &NotSupportedError{})
+			t.Errorf("Delete() actual = (%v), expected (%v)", actualErr, &NotSupportedError{})
+		}
+	})
+
+	t.Run("Delete is ignored with IGNORE_DOCKER_LOGIN", func(t *testing.T) {
+		t.Setenv(envIgnoreLogin, "1")
+		actualErr := e.Delete("")
+		if actualErr != nil {
+			t.Errorf("Delete() actual = (%v), expected (%v)", actualErr, nil)
 		}
 	})
 
 	t.Run("List is not supported", func(t *testing.T) {
 		_, actualErr := e.List()
 		if !errors.Is(actualErr, &NotSupportedError{}) {
-			t.Errorf("Add() actual = (%v), expected (%v)", actualErr, &NotSupportedError{})
+			t.Errorf("List() actual = (%v), expected (%v)", actualErr, &NotSupportedError{})
 		}
 	})
 }