Update docker-compose example to version 3

Adds "depends_on"

install.rst

@@ -203,6 +203,8 @@ To avoid having to install the database at all, you can run both it and the serv
 
   # docker-compose.yml
 
+  version: '3'
+  services:
     server:
       image: postgrest/postgrest
       ports:
@@ -213,7 +215,8 @@ To avoid having to install the database at all, you can run both it and the serv
         PGRST_DB_URI: postgres://app_user:password@db:5432/app_db
         PGRST_DB_SCHEMA: public
         PGRST_DB_ANON_ROLE: app_user
-
+      depends_on:
+        - db
     db:
       image: postgres
       ports: