Simplify init scripts

Single script init.sh which performs the following tasks:
- Creates required directory structure
- Activates python venv (if required)
- Waits for database connection
- Runs command

docker/docker-compose.dev.yml

@@ -35,7 +35,7 @@ services:
         build:
             context: .
             target: dev
-        entrypoint: /home/inventree/docker/start_dev_worker.sh
+        command: invoke worker
         depends_on:
             - inventree-dev-server
         volumes: