Publish Docker image to GHCR on main builds

Add a Docker workflow that builds on PRs and pushes to
ghcr.io/chewbaccalakis/rfid-database on main (latest + sha) and v* tags.
Drop the redundant docker-build job from CI.

docker-compose.yml

@@ -1,9 +1,13 @@
 services:
   rfid-database:
+    # Local build (default):
     build:
       context: .
       dockerfile: Dockerfile
     image: rfid-database:local
+    # Or pull from GHCR instead of building:
+    # image: ghcr.io/chewbaccalakis/rfid-database:latest
+    # pull_policy: always
     container_name: rfid-database
     restart: unless-stopped
     ports: