fix(examples): mount config-compose.yaml so docker compose up reaches NATS

The examples stack mounted config.yaml (server_url=localhost:4222) into the
in-container opa service, so localhost resolved to the OPA container and the
plugin couldn't reach the `nats` service. Mount config-compose.yaml
(nats://nats:4222) instead and drop the unused NATS_URL env.

Addresses review comments:
- #4 (comment) (@zeevmoney)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: omer9564

examples/opa-nats/docker-compose.yaml

@@ -121,10 +121,12 @@ services:
     ports:
       - "8181:8181"
     volumes:
-      - ./config.yaml:/config/config.yaml
+      # In-compose OPA must reach NATS via the service name, so mount the
+      # compose config (server_url=nats://nats:4222), NOT config.yaml's
+      # host-friendly localhost default — otherwise `localhost:4222` resolves to
+      # the OPA container itself and the plugin can't reach NATS.
+      - ./config-compose.yaml:/config/config.yaml
       - ./rego:/policies
-    environment:
-      - NATS_URL=nats://nats:4222
     networks:
       - nats-net