Fix postgres error FATAL: role "root" does not exist

Related Github issue: peter-evans/docker-compose-healthcheck#17 (comment)

Author: eamonburns

docker-compose.yaml

@@ -13,7 +13,7 @@ services:
     environment:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready"]
+      test: ["CMD-SHELL", "pg_isready -d ${POSTGRES_DBNAME} -U postgres"]
       interval: 5s
       timeout: 5s
       retries: 10