Add infra/docker-compose.yml for shared postgres stack

Move compose config from README into its own file; README now references it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/docker-compose.yml

@@ -0,0 +1,16 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    restart: unless-stopped
+    ports:
+      - "5433:5432"
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-postgres}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?POSTGRES_PASSWORD is required}
+    volumes:
+      - /volume2/docker/shared/pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-postgres}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5