create-app: add sign-in resolver example

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/nasty-paws-move.md

@@ -0,0 +1,5 @@
+---
+'@backstage/create-app': patch
+---
+
+Tweaked template to provide an example and guidance for how to configure sign-in in `packages/backend/src/plugins/auth.ts`. There is no need to add this to existing apps, but for more information about sign-in configuration, see https://backstage.io/docs/auth/identity-resolver.

packages/create-app/templates/default-app/packages/backend/src/plugins/auth.ts

@@ -1,4 +1,8 @@
-import { createRouter } from '@backstage/plugin-auth-backend';
+import {
+  createRouter,
+  providers,
+  defaultAuthProviderFactories,
+} from '@backstage/plugin-auth-backend';
 import { Router } from 'express';
 import { PluginEnvironment } from '../types';
 
@@ -11,5 +15,22 @@ export default async function createPlugin(
     database: env.database,
     discovery: env.discovery,
     tokenManager: env.tokenManager,
+    providerFactories: {
+      ...defaultAuthProviderFactories,
+
+      // This overrides the default GitHub auth provider with a custom one.
+      // Since the options are empty it will behave just like the default
+      // provider, but if you uncomment the `signIn` section you will enable
+      // sign-in via GitHub. This particular configuration uses a resolver
+      // that matches the username to the user entity name. See the auth
+      // documentation for more details on how to enable and customize sign-in:
+      //
+      //   https://backstage.io/docs/auth/identity-resolver
+      github: providers.github.create({
+        // signIn: {
+        //   resolver: providers.github.resolvers.usernameMatchingUserEntityName(),
+        // },
+      }),
+    },
   });
 }