Fixed docker-compose health check (#13)

One line change

## Checklist
- [x] My PR name is descriptive and in imperative tense
- [x] My commit messages are descriptive and in imperative tense. My
commits are atomic and trivial commits are squashed or fixup'd into
non-trivial commits
- [x] I have run the appropriate linter(s)
- [x] I have requested a review from the PL, as well as other devs who
have background knowledge on this PR or who will be building on top of
this PR

docker-compose.yml

@@ -36,7 +36,7 @@ services:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
       POSTGRES_DB: ${POSTGRES_DATABASE}
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER}"]
       interval: 5s
       timeout: 5s
       retries: 5