fix: add environment variable POSTGRES_USER (#20786)

docker/.env.example

@@ -816,6 +816,8 @@ TEXT_GENERATION_TIMEOUT_MS=60000
 # ------------------------------
 
 PGUSER=${DB_USERNAME}
+# The name of the default postgres user.
+POSTGRES_USER=${DB_USERNAME}
 # The password for the default postgres user.
 POSTGRES_PASSWORD=${DB_PASSWORD}
 # The name of the default postgres database.