Add note about v4 -> v5 later migration

compose.yml

@@ -27,7 +27,7 @@ services:
         - "traefik.http.routers.invoiceninja.tls.certresolver=${LETS_ENCRYPT_ENV}"
 
   invoiceninja:
-    image: "invoiceninja/invoiceninja:5.0.4"
+    image: "invoiceninja/invoiceninja:4.5.18"
     volumes:
       - "public:/var/www/app/public"
       - "storage:/var/www/app/storage"
@@ -45,6 +45,9 @@ services:
       - APP_URL=${DOMAIN}
       - DB_DATABASE=ninja
       - DB_HOST=mariadb
+      # Note(decentral1se): this isnt supported until the v5 tag stabilises so
+      # when you end up on the initial /setup URL, you need to copy in the
+      # password. Once v5 is out, this will be seemless
       - DB_PASSWORD_FILE=/run/secrets/db_passwd
       - DB_STRICT=false
       - DB_TYPE=mysql