Implement manager worker architecture

[PhilippMatthes]

• Add postgis to image as background process
• Small adjustments to README
• Add worker and sync env vars / settings to control which endpoints are enabled and how the service behaves
• Add healthcheck and status view for Docker healthcheck
• Send news articles and categories to workers when the manager makes updates
• Ensure FCM message is not sent with dev environment
• Add sync command that is executed once on startup of workers to sync with manager
• Add sync get resource for manager that provides the necessary data for worker sync
• Implement minimalistic compose setup with 2 workers and 1 manager
• Remove outer-inner nginx setup
• Run two servers in parallel, one listening on the internal Docker network for sync, and one for external requests

[PhilippMatthes]

Needs some more work