fix: correct postgres docker healthcheck

since this is only for development we can use the default user instead
of expecting the user to be set via env variable

Author: janz93

docker-compose.with_db.yml

@@ -24,7 +24,7 @@ services:
     networks:
       - ebicsbox_network
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U $POSTGRES_USER"]
+      test: ["CMD-SHELL", "pg_isready", "-U", "postgres"]
       interval: 10s
       timeout: 5s
       retries: 5
@@ -40,7 +40,7 @@ services:
     networks:
       - ebicsbox_network
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U $POSTGRES_USER"]
+      test: ["CMD-SHELL", "pg_isready", "-U", "postgres"]
       interval: 10s
       timeout: 5s
       retries: 5