docs: point to pre-built Docker Hub image in README and docker-compose

- docker-compose.yml: changed from build: . to image: droideparanoico/dostalgia
- README: simplified quick start to pull instead of build
- README: added link to Docker Hub repo with CI note

docker-compose.yml

@@ -1,6 +1,8 @@
 services:
   dostalgia:
-    build: .
+    image: droideparanoico/dostalgia
+    # Uncomment to build from source instead of using the pre-built image:
+    # build: .
     container_name: dostalgia
     ports:
       - "8765:8765"