Update auth for preview environments

auth.ts

@@ -1,6 +1,14 @@
 import NextAuth from 'next-auth'
 import GitHub from 'next-auth/providers/github'
+import CredentialsProvider from 'next-auth/providers/credentials'
 
+// We default to using GitHub for authentication for local development and production.
+// On Preview deployments, we use a dummy credentials provider. This allows folks to easily
+// test the app without having to create a custom GitHub OAuth app or change the callback URL
+// just to test the application on previews.
+
+// We have a custom /sign-in page for non-preview environments. In preview environments, the user
+// will be redirected to /api/auth/signin instead.
 export const {
   handlers: { GET, POST },
   auth,
@@ -8,7 +16,30 @@ export const {
   // @ts-ignore
 } = NextAuth({
   // @ts-ignore
-  providers: [GitHub],
+  providers: [
+    process.env.VERCEL_ENV === 'preview'
+      ? CredentialsProvider({
+          name: 'Credentials',
+          credentials: {
+            username: {
+              label: 'Username',
+              type: 'text',
+              placeholder: 'jsmith'
+            },
+            password: { label: 'Password', type: 'password' }
+          },
+          async authorize(credentials) {
+            return {
+              id: 1,
+              name: 'J Smith',
+              email: 'jsmith@example.com',
+              picture: 'https://i.pravatar.cc/150?u=jsmith@example.com'
+            } as any
+          }
+        })
+      : GitHub
+  ],
+  debugger: true,
   callbacks: {
     // @ts-ignore
     jwt: async ({ token, profile }) => {
@@ -21,9 +52,14 @@ export const {
     // @ts-ignore
     authorized({ auth }) {
       return !!auth?.user
-    }
+    },
+    trustHost: true
   },
-  pages: {
-    signIn: '/sign-in'
-  }
+  ...(process.env.VERCEL_ENV === 'preview'
+    ? {
+        pages: {
+          signIn: '/sign-in'
+        }
+      }
+    : {})
 })