Add healthcheck to Postgres & make API depend on it

Now the backend only starts, when the Postgres instance is running & healthy.

docker-compose.yml

@@ -4,6 +4,11 @@ services:
     container_name: postgres
     image: 'postgres:15'
     restart: always
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
     ports:
       - '5432:5432'
     env_file:
@@ -26,7 +31,8 @@ services:
     links:
       - postgres
     depends_on:
-      - postgres
+      postgres:
+        condition: service_healthy
     networks:
       - default