Fix initial creation of s3 bucket

The current docker-compose code will not create the bucket - at least not so far as I tested locally (could be a local problem). The problem with it is that the createbucket container starts working before the minio is ready (so the combination of the heathcheck and the depdends_on value doesn't work).

I fxied it locally and test it currently on the !233 (merged) branch, so I think I can provide the patch for this in general as well.