Update Postgres backup docs for ARM32

docs/maintenance-postgres.md

@@ -64,6 +64,8 @@ pg_dumpall -h matrix-postgres \
 
 If you are using an [external Postgres server](configuring-playbook-external-postgres.md), the above command will not work, because the credentials file (`/matrix/postgres/env-postgres-psql`) is not available.
 
+If your server is on the ARM32 [architecture](alternative-architectures.md), you may need to remove the `-alpine` suffix from the image name in the command above.
+
 Restoring a backup made this way can be done by [importing it](importing-postgres.md).