Added information about database file mapping to docker installation and use this in example docker-compose.yaml

This fixes issue #134 and #127

docs/docker/docker-compose.yaml

@@ -9,5 +9,9 @@ services:
       # By default
       - ./uploads:/var/www/html/uploads
       - ./public_media:/var/www/html/public/media
+      - ./db:/var/www/html/var/db
     restart: unless-stopped
-    image: jbtronics/part-db1:master
+    image: jbtronics/part-db1:master
+    environment:
+      # Put SQLite database in our mapped folder. You can configure some other kind of database here too.
+      - DATABASE_URL=sqlite:///%kernel.project_dir%/var/db/app.db