Fix "fatal role postgres doesn't exist" (#2296)

* Fix "fatal role postgres doesn't exist" 

Fix a frequent error with recent postgres docker images:
`FATAL:  role "postgres" does not exist`

* Use $$VAR so it's expanded by the shell, not docker

docker-compose.yml

@@ -12,7 +12,7 @@ services:
       POSTGRES_PASSWORD: kemal
       POSTGRES_USER: kemal
     healthcheck:
-      test: ["CMD", "pg_isready", "-U", "postgres"]
+      test: ["CMD-SHELL", "pg_isready -U $$POSTGRES_USER"]
   invidious:
     build:
       context: .