Our tagging strategy is described in this post: Branches and Docker images
All images corresponds to current up-to date versions, alpha is the up to date dev (unstable) version (to be used for preview testing only), beta is the up-to-date semi-stable version (to be used for validation testing), latest is the up-to-date stable version (to be used for production)
Tagging with a release number (e.g. 4.0.P21) would only make sense for the stable images but, anyway, we expect our customers to always be up to date.
As a matter of fact, Simplicité has a continuous delivery model, there is absolutely no good reason to stick to a given previous version. Updating to the up-to-date version is straightforward and it is infinitely more risky to keep using a previous version than to upgrade.
As a consequence, even if we put a tag with a explicit version/revision number on the stable images we will remove previous images to be sure no one would use them for bad reasons…