fix: Add DB_DATA_LOCATION config. (#13754)

Add DB_DATA_LOCATION config.

Posted [on reddit](https://www.reddit.com/r/unRAID/comments/1f8wq45/immich_install_via_docker_compose_database_cannot/).

docs/docs/install/unraid.md

@@ -77,6 +77,7 @@ alt="Select Plugins > Compose.Manager > Add New Stack > Label it Immich"
 7.  Paste the entire contents of the [Immich example.env](https://github.com/immich-app/immich/releases/latest/download/example.env) file into the Unraid editor, then **before saving** edit the following:
 
     - `UPLOAD_LOCATION`: Create a folder in your Images Unraid share and place the **absolute** location here > For example my _"images"_ share has a folder within it called _"immich"_. If I browse to this directory in the terminal and type `pwd` the output is `/mnt/user/images/immich`. This is the exact value I need to enter as my `UPLOAD_LOCATION`
+    - `DB_DATA_LOCATION`: Change this to use an Unraid share (preferably a cache pool, e.g. `/mnt/user/appdata`). If left at default it will try to use Unraid's `/boot/config/plugins/compose.manager/projects/[stack_name]/postgres` folder which it doesn't have permissions to, resulting in this container continuously restarting.
 
       <img
       src={require('./img/unraid05.webp').default}