docs(docker): add docker-compose example using the prebuilt image

Mirrors docker-compose.yml's postgres/migrate/broker stack but pulls
git.warky.dev/wdevs/pgsql-broker:latest instead of building from
source, so it needs no local Go toolchain or Dockerfile.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.prebuilt.yml

@@ -0,0 +1,46 @@
+services:
+  postgres:
+    image: docker.io/library/postgres:16-alpine
+    environment:
+      POSTGRES_DB: broker
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres -d broker"]
+      interval: 2s
+      timeout: 3s
+      retries: 30
+    restart: unless-stopped
+
+  # One-shot: applies migrations and creates/rotates the least-privilege
+  # broker_admin/broker_runtime/broker_enqueue roles, then exits. The
+  # broker service below only starts once this completes successfully.
+  migrate:
+    image: git.warky.dev/wdevs/pgsql-broker:latest
+    depends_on:
+      postgres:
+        condition: service_healthy
+    volumes:
+      - ./broker.docker.yaml:/etc/pgsql-broker/broker.yaml:ro
+    environment:
+      PGUSER: postgres
+      PGPASSWORD: ${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}
+      BROKER_ADMIN_PASSWORD: ${BROKER_ADMIN_PASSWORD:?set BROKER_ADMIN_PASSWORD in .env}
+      BROKER_RUNTIME_PASSWORD: ${BROKER_RUNTIME_PASSWORD:?set BROKER_RUNTIME_PASSWORD in .env}
+      BROKER_ENQUEUE_PASSWORD: ${BROKER_ENQUEUE_PASSWORD:?set BROKER_ENQUEUE_PASSWORD in .env}
+    command: ["install", "--with-roles"]
+    restart: "no"
+
+  broker:
+    image: git.warky.dev/wdevs/pgsql-broker:latest
+    depends_on:
+      migrate:
+        condition: service_completed_successfully
+    volumes:
+      - ./broker.docker.yaml:/etc/pgsql-broker/broker.yaml:ro
+    restart: unless-stopped
+
+volumes:
+  postgres-data: