Add postgres major version upgrade script

compose.yml

@@ -52,13 +52,18 @@ services:
     #   start_period: 1m
 
   db:
-    image: postgres:13-alpine
+    image: postgres:13
     networks:
       - internal
     secrets:
       - db_password
     volumes:
       - 'postgresql_data:/var/lib/postgresql/data'
+    configs:
+      - source: db_entrypoint
+        target: /docker-entrypoint.sh
+        mode: 0555
+    entrypoint: /docker-entrypoint.sh
     environment:
       - POSTGRES_HOST_AUTH_METHOD=trust
       - POSTGRES_USER=discourse
@@ -120,3 +125,9 @@ networks:
   proxy:
     external: true
   internal:
+
+configs:
+  db_entrypoint:
+    name: ${STACK_NAME}_db_entrypoint_${DB_ENTRYPOINT_VERSION}
+    file: entrypoint.postgres.sh.tmpl
+    template_driver: golang