On "production" instances, createbuckets keeps waiting

From recent change in this file:

https://github.com/codalab/codabench/blob/develop/docker-compose.yml

The line 76 is stuck and returns this every five seconds:

createbuckets_1   | + echo ...waiting...
createbuckets_1   | ...waiting...
createbuckets_1   | + sleep 5

This happens when there is a MinIO associated to the instance (so the MinIO access key and secret key are defined) but the MinIO is external to the VM of the instance.

A simple fix is to cap to a maximum number of retries, or to add a condition depending on the configuration.

Warning: after changing this, it should be tested both in a "production" server and in a fresh local install