This warning does not exist at this endpoint. Are you sure you exported correctly?
What’s inside the volume is not necessarily “backup”. It’s usually good to clean the volume for each upgrade so you do not risk upgrading to incompatible versions. So you should export and then bulkload the exported RDF.
This happens because we change the way Alpha Server starts. You should use “Dgraph Alpha” instead of “Dgraph Server”. You must update your docker-compose tho.