Improve docker setup

- add .dockerignore
- add entrypoint.sh to switch to user after initial setup
- downgrade importlib-metadata for Celery compatibility
- add TZ environment variable for consistent Celery timestamps
- fix Celery setting deprecation warnings

pyproject.toml

@@ -13,6 +13,7 @@ amqp = "^5.1.1"
 Jinja2 = "^3.1.2"
 redis = "^4.5.1"
 pytz = "^2022.7.1"
+importlib-metadata = "<5"
 
 [tool.poetry.group.dev.dependencies]
 coverage = "^7.1.0"