From what @laurent has said on this forum, the server software is currently in beta. There are no "release" versions so all published images are beta versions and should be selected in the docker-compose file using its specific version, eg server:2.2.8-beta. When the server software is considered stable enough to be "released" that image version will additionally be tagged :latest.
I believe that from then on :latest will indicate the latest Release version with :x.x.x-beta (or possibly something similar) indicating a Pre-release. So from then onwards if you use the :latest tag you will only get the latest image considered suitable for use.
It's similar to the way the desktop client software uses Release and Pre-Release.
@laurent has mentioned that the current :latest tag was issued in error to Server v2.1.1-beta, hence it has not been used since as there is currently no release version. That being said, from what I have read the first release version of the server software is likely not that far off.