add database upgrade instructions

docker-compose.yml

@@ -32,7 +32,12 @@ services:
       - cannery-db
 
   cannery-db:
-    image: postgres:13
+    image: postgres:17
+    # To upgrade your database from a previous version, replace the existing
+    # image with the following line. Once the container has been booted and your
+    # database updated, you can revert back to the original `postgres:17` image
+    # if you'd like
+    # image: pgautoupgrade/pgautoupgrade:17-alpine
     container_name: cannery-db
     environment:
       - POSTGRES_USER="postgres"