build: add NEXTAUTH_URL_INTERNAL to example env (#108)

Co-authored-by: Matti Nannt <mail@matthiasnannt.com>

.env.example

@@ -9,8 +9,12 @@
 
 NEXTAUTH_SECRET=RANDOM_STRING
 
+# Set this to your public-facing URL, e.g., https://example.com
 NEXTAUTH_URL=http://localhost:3000
 
+# This should always be localhost:3000 (or whatever port your app is running on)
+NEXTAUTH_URL_INTERNAL=http://localhost:3000
+
 DATABASE_URL='postgresql://postgres:postgres@localhost:5432/snoopforms?schema=public'
 # For Docker Compose Production Setup use this Database URL:
 # DATABASE_URL='postgresql://postgres:postgres@postgres:5432/snoopforms?schema=public'
@@ -74,4 +78,4 @@ SMTP_PASSWORD=smtpPassword
 # We can't identify you, or your users and only receive the number of submissions on your instance.
 # You help us a lot, if you leave this activated.
 # If you still want to opt-out, uncomment the next line.
-# TELEMETRY_DISABLED=1
+# TELEMETRY_DISABLED=1