add information on DISABLE_REGISTRATION

Author: ryanwinter

pages/configuration/reference.mdx

@@ -36,6 +36,16 @@ eg: `http://postiz.example.lan:3000`
 
 If running everything in the same host/container: `http://localhost:3000`
 
+## Optional settings
+
+Refer to the [example postiz.env file](https://raw.githubusercontent.com/gitroomhq/postiz-app/main/.env.example) for additional configuration options.
+
+### DISABLE_REGISTRATION
+
+eg: `true`
+
+This will only allow a single user registration, and then after that the sign-up page will be disabled. Useful for self-hosting where you want complete control over users.
+
 ## Social Media keys
 
 See the "providers" section of the documentation for the provider you want to configure,

pages/installation/docker-compose.mdx

@@ -49,6 +49,7 @@ services:
       REDIS_URL: "redis://postiz-redis:6379"
       BACKEND_INTERNAL_URL: "http://localhost:3000"
       IS_GENERAL: "true" # Required for self-hosting.
+      DISABLE_REGISTRATION: "true" # Only allow single registration, then disable signup
       # The container images are pre-configured to use /uploads for file storage.
       # You probably should not change this unless you have a really good reason!
       STORAGE_PROVIDER: "local"