Update docker compose instructions

README.md

@@ -54,7 +54,9 @@ it as needed. Run it with
 ```
 docker compose up -d
 ```
-
+If you're using a custom location for the `db_url`, make sure to make that file
+before running the docker image, as otherwise a directory will be created in its
+place, resulting in an error.
 ## Building from source
 Clone this repository
 ```

docker-compose.yml

@@ -8,6 +8,7 @@ services:
             - 4567:4567
         environment:
             # Change if you want to mount the database somewhere else
+            # In this case, you can get rid of the db volume below
             # - db_url=/urls.sqlite
             - username=admin
             - password=$3CuReP4S$W0rD